[be] Move phi aliasing to InferAliasForPhis

InferMutableRangesForAlias is about extending the mutable ranges, not for 
updating the alias sets. Let's refactor this into a separate pass. 

InferMutableRangesForAlias was iterating over alias sets and not the HIR so this 
refactor isn't costing us any additional perf cost (in terms of an extra 
iteration over the HIR).

compiler/forget/src/Inference/InferAliasForPhis.ts

@@ -0,0 +1,26 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+import { HIRFunction, Identifier } from "../HIR/HIR";
+import DisjointSet from "../Utils/DisjointSet";
+
+export function inferAliasForPhis(
+  func: HIRFunction,
+  aliases: DisjointSet<Identifier>
+) {
+  for (const [_, block] of func.body.blocks) {
+    for (const phi of block.phis) {
+      const isPhiMutatedAfterCreation: boolean =
+        phi.id.mutableRange.end >
+        (block.instructions.at(0)?.id ?? block.terminal.id);
+      if (isPhiMutatedAfterCreation) {
+        for (const [, operand] of phi.operands) {
+          aliases.union([phi.id, operand]);
+        }
+      }
+    }
+  }
+}

compiler/forget/src/Inference/InferMutableRanges.ts

@@ -7,6 +7,7 @@
 
 import { HIRFunction, Identifier } from "../HIR/HIR";
 import { inferAliases } from "./InferAlias";
+import { inferAliasForPhis } from "./InferAliasForPhis";
 import { inferAliasForStores } from "./InferAliasForStores";
 import { inferMutableLifetimes } from "./InferMutableLifetimes";
 import { inferMutableRangesForAlias } from "./InferMutableRangesForAlias";
@@ -27,6 +28,9 @@ export function inferMutableRanges(ir: HIRFunction) {
     // Update aliasing information of fields
     inferAliasForStores(ir, aliases);
 
+    // Update aliasing information of phis
+    inferAliasForPhis(ir, aliases);
+
     const nextAliases = aliases.canonicalize();
     if (areEqualMaps(prevAliases, nextAliases)) {
       break;

compiler/forget/src/Inference/InferMutableRangesForAlias.ts

@@ -31,17 +31,4 @@ export function inferMutableRangesForAlias(
       }
     }
   }
-
-  for (const [_, block] of fn.body.blocks) {
-    for (const phi of block.phis) {
-      const isPhiMutatedAfterCreation: boolean =
-        phi.id.mutableRange.end >
-        (block.instructions.at(0)?.id ?? block.terminal.id);
-      if (isPhiMutatedAfterCreation) {
-        for (const [, operand] of phi.operands) {
-          aliases.union([phi.id, operand]);
-        }
-      }
-    }
-  }
 }