Add new test cases for SSA

Add these as a separate commit so we can see how later PRs in the stack change 
them

compiler/forget/src/__tests__/fixtures/hir/ssa-leave-case.expect.md

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+
+## Input
+
+```javascript
+function Component(props) {
+  let x = [];
+  let y;
+  if (props.p0) {
+    x.push(props.p1);
+    y = x;
+  }
+  return (
+    <Component>
+      {x}
+      {y}
+    </Component>
+  );
+}
+
+```
+
+## Code
+
+```javascript
+function Component(props) {
+  const $ = React.unstable_useMemoCache();
+  const c_0 = $[0] !== props.p0;
+  const c_1 = $[1] !== props.p1;
+  let x;
+  let y$0;
+  if (c_0 || c_1) {
+    x = [];
+    const y = undefined;
+    y$0 = y;
+    if (props.p0) {
+      x.push(props.p1);
+      const y$1 = x;
+      y$0 = y$1;
+    }
+    $[0] = props.p0;
+    $[1] = props.p1;
+    $[2] = x;
+    $[3] = y$0;
+  } else {
+    x = $[2];
+    y$0 = $[3];
+  }
+  const c_4 = $[4] !== x;
+  const c_5 = $[5] !== y$0;
+  let t0;
+  if (c_4 || c_5) {
+    t0 = (
+      <Component>
+        {x}
+        {y$0}
+      </Component>
+    );
+    $[4] = x;
+    $[5] = y$0;
+    $[6] = t0;
+  } else {
+    t0 = $[6];
+  }
+  return t0;
+}
+
+```
+      

compiler/forget/src/__tests__/fixtures/hir/ssa-leave-case.js

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+function Component(props) {
+  let x = [];
+  let y;
+  if (props.p0) {
+    x.push(props.p1);
+    y = x;
+  }
+  return (
+    <Component>
+      {x}
+      {y}
+    </Component>
+  );
+}

compiler/forget/src/__tests__/fixtures/hir/ssa-renaming-with-mutation.expect.md

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+
+## Input
+
+```javascript
+function foo(props) {
+  let x = [];
+  x.push(props.bar);
+  if (props.cond) {
+    x = {};
+    x = [];
+    x.push(props.foo);
+  }
+  mut(x);
+  return x;
+}
+
+```
+
+## Code
+
+```javascript
+function foo(props) {
+  const $ = React.unstable_useMemoCache();
+  const c_0 = $[0] !== props.bar;
+  let x;
+  if (c_0) {
+    x = [];
+    x.push(props.bar);
+    $[0] = props.bar;
+    $[1] = x;
+  } else {
+    x = $[1];
+  }
+  const c_2 = $[2] !== props.cond;
+  const c_3 = $[3] !== props.foo;
+  let x$0;
+  if (c_2 || c_3) {
+    x$0 = x;
+    if (props.cond) {
+      const x$1 = [];
+      x$1.push(props.foo);
+      x$0 = x$1;
+    }
+
+    mut(x$0);
+    $[2] = props.cond;
+    $[3] = props.foo;
+    $[4] = x$0;
+  } else {
+    x$0 = $[4];
+  }
+  return x$0;
+}
+
+```
+      

compiler/forget/src/__tests__/fixtures/hir/ssa-renaming-with-mutation.js

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+function foo(props) {
+  let x = [];
+  x.push(props.bar);
+  if (props.cond) {
+    x = {};
+    x = [];
+    x.push(props.foo);
+  }
+  mut(x);
+  return x;
+}

compiler/forget/src/__tests__/fixtures/hir/ssa-renaming.expect.md

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+
+## Input
+
+```javascript
+function foo(props) {
+  let x = [];
+  x.push(props.bar);
+  if (props.cond) {
+    x = {};
+    x = [];
+    x.push(props.foo);
+  }
+  return x;
+}
+
+```
+
+## Code
+
+```javascript
+function foo(props) {
+  const $ = React.unstable_useMemoCache();
+  const c_0 = $[0] !== props.bar;
+  let x;
+  if (c_0) {
+    x = [];
+    x.push(props.bar);
+    $[0] = props.bar;
+    $[1] = x;
+  } else {
+    x = $[1];
+  }
+  const c_2 = $[2] !== props.cond;
+  const c_3 = $[3] !== props.foo;
+  let x$0;
+  if (c_2 || c_3) {
+    x$0 = x;
+    if (props.cond) {
+      const c_5 = $[5] !== props.foo;
+      let x$1;
+      if (c_5) {
+        x$1 = [];
+        x$1.push(props.foo);
+        $[5] = props.foo;
+        $[6] = x$1;
+      } else {
+        x$1 = $[6];
+      }
+      x$0 = x$1;
+    }
+    $[2] = props.cond;
+    $[3] = props.foo;
+    $[4] = x$0;
+  } else {
+    x$0 = $[4];
+  }
+  return x$0;
+}
+
+```
+      

compiler/forget/src/__tests__/fixtures/hir/ssa-renaming.js

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+function foo(props) {
+  let x = [];
+  x.push(props.bar);
+  if (props.cond) {
+    x = {};
+    x = [];
+    x.push(props.foo);
+  }
+  return x;
+}