Fixture tests for 2 cases of expressions that produce incorrect output

They're fairly related, but I figured it's worth keeping more examples. 

- For the `while` example we need to codegen into a single expression. - For the 
expression with contained assignment we need to either keep the SSA ids around 
or re-create a similar expression during codegen.

compiler/forget/src/__tests__/fixtures/hir/_bug_expression-with-assignment.expect.md

@@ -0,0 +1,60 @@
+
+## Input
+
+```javascript
+function f() {
+  let x = 1;
+  // BUG: `x` has different values within this expression. Currently, the
+  // assignment is evaluated too early.
+  return x + (x = 2) + x;
+}
+
+```
+
+## HIR
+
+```
+bb0:
+  [1] Let mutate x$4_@0 = 1
+  [2] Reassign mutate x$5_@1 = 2
+  [3] Const mutate $6_@2 = Binary read x$5_@1 + read x$5_@1
+  [4] Const mutate $7_@3 = Binary read $6_@2 + read x$5_@1
+  [5] Return read $7_@3
+scope2 [3:4]:
+ - read x$5_@1
+ - read x$5_@1
+scope3 [4:5]:
+ - read $6_@2
+ - read x$5_@1
+```
+
+### CFG
+
+```mermaid
+flowchart TB
+  %% Basic Blocks
+  subgraph bb0
+    bb0_instrs["
+      [1] Let mutate x$4_@0 = 1
+      [2] Reassign mutate x$5_@1 = 2
+      [3] Const mutate $6_@2 = Binary read x$5_@1 + read x$5_@1
+      [4] Const mutate $7_@3 = Binary read $6_@2 + read x$5_@1
+    "]
+    bb0_instrs --> bb0_terminal(["Return read $7_@3"])
+  end
+
+  %% Jumps
+  %% empty
+```
+
+## Code
+
+```javascript
+function f$0() {
+  let x$1 = 1;
+  x$1 = 2;
+  return x$1 + x$1 + x$1;
+}
+
+```
+      

compiler/forget/src/__tests__/fixtures/hir/_bug_expression-with-assignment.js

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+function f() {
+  let x = 1;
+  // BUG: `x` has different values within this expression. Currently, the
+  // assignment is evaluated too early.
+  return x + (x = 2) + x;
+}

compiler/forget/src/__tests__/fixtures/hir/_bug_while-with-assignment.expect.md

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+
+## Input
+
+```javascript
+function f(reader) {
+  const queue = [1, 2, 3];
+  let value = 0;
+  let sum = 0;
+  // BUG: we need to codegen the complex test expression
+  while ((value = queue.pop()) != null) {
+    sum += value;
+  }
+  return sum;
+}
+
+```
+
+## HIR
+
+```
+bb0:
+  [1] Const mutate $11_@0 = 1
+  [2] Const mutate $12_@1 = 2
+  [3] Const mutate $13_@2 = 3
+  [4] Const mutate queue$14_@3[0:14] = Array [read $11_@0, read $12_@1, read $13_@2]
+  [5] Let mutate value$15_@4 = 0
+  [6] Let mutate sum$16_@3[0:14] = 0
+  [7] While test=bb1 loop=bb3 fallthrough=bb2
+bb1:
+  predecessor blocks: bb0 bb3
+  sum$21_@3[0:14]: phi(bb0: sum$16_@3, bb3: sum$22_@3)
+  [8] Reassign mutate value$18_@3[0:14] = Call mutate queue$14_@3.pop()
+  [9] Const mutate $19_@6 = null
+  [10] Const mutate $20_@7[10:14] = Binary read value$18_@3 != read $19_@6
+  [11] If (read $20_@7) then:bb3 else:bb2
+bb3:
+  predecessor blocks: bb1
+  [12] Reassign mutate sum$22_@3[0:14] = Binary read sum$21_@3 + read value$18_@3
+  [13] Goto(Continue) bb1
+bb2:
+  predecessor blocks: bb1
+  [14] Return read sum$21_@3
+scope7 [10:14]:
+ - read $19_@6
+```
+
+### CFG
+
+```mermaid
+flowchart TB
+  %% Basic Blocks
+  subgraph bb0
+    bb0_instrs["
+      [1] Const mutate $11_@0 = 1
+      [2] Const mutate $12_@1 = 2
+      [3] Const mutate $13_@2 = 3
+      [4] Const mutate queue$14_@3[0:14] = Array [read $11_@0, read $12_@1, read $13_@2]
+      [5] Let mutate value$15_@4 = 0
+      [6] Let mutate sum$16_@3[0:14] = 0
+    "]
+    bb0_instrs --> bb0_terminal(["While"])
+  end
+  subgraph bb1
+    bb1_instrs["
+      [8] Reassign mutate value$18_@3[0:14] = Call mutate queue$14_@3.pop()
+      [9] Const mutate $19_@6 = null
+      [10] Const mutate $20_@7[10:14] = Binary read value$18_@3 != read $19_@6
+    "]
+    bb1_instrs --> bb1_terminal(["If (read $20_@7)"])
+  end
+  subgraph bb3
+    bb3_instrs["
+      [12] Reassign mutate sum$22_@3[0:14] = Binary read sum$21_@3 + read value$18_@3
+    "]
+    bb3_instrs --> bb3_terminal(["Goto"])
+  end
+  subgraph bb2
+    bb2_terminal(["Return read sum$21_@3"])
+  end
+
+  %% Jumps
+  bb0_terminal -- "test" --> bb1
+  bb0_terminal -- "loop" --> bb3
+  bb0_terminal -- "fallthrough" --> bb2
+  bb1_terminal -- "then" --> bb3
+  bb1_terminal -- "else" --> bb2
+  bb3_terminal --> bb1
+
+```
+
+## Code
+
+```javascript
+function f$0(reader$1) {
+  const queue$2 = [1, 2, 3];
+  let value$6 = 0;
+  let sum$7 = 0;
+  value$6 = queue$2.pop();
+  bb2: while (value$6 != null) {
+    sum$7 = sum$7 + value$6;
+  }
+
+  return sum$7;
+}
+
+```
+      

compiler/forget/src/__tests__/fixtures/hir/_bug_while-with-assignment.js

@@ -0,0 +1,10 @@
+function f(reader) {
+  const queue = [1, 2, 3];
+  let value = 0;
+  let sum = 0;
+  // BUG: we need to codegen the complex test expression
+  while ((value = queue.pop()) != null) {
+    sum += value;
+  }
+  return sum;
+}