[InstCombine] Use a cl::opt to control calls to getOrEnforceKnownAlignment in LoadInst and StoreInst

This is in preparation for the InferAlignment pass which handles
inferring alignment for instructions separately. It is better to handle
this as a separate pass as inferring alignment is quite costly, and
InstCombine running multiple times in the pass pipeline makes it even
more so.

Differential Revision: https://reviews.llvm.org/D158527

Author: dc03

llvm/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp

@@ -36,6 +36,8 @@ static cl::opt<unsigned> MaxCopiedFromConstantUsers(
     cl::desc("Maximum users to visit in copy from constant transform"),
     cl::Hidden);
 
+extern cl::opt<bool> EnableInferAlignmentPass;
+
 /// isOnlyCopiedFromConstantMemory - Recursively walk the uses of a (derived)
 /// pointer to an alloca.  Ignore any reads of the pointer, return false if we
 /// see any stores or other unknown uses.  If we see pointer arithmetic, keep
@@ -1048,11 +1050,13 @@ Instruction *InstCombinerImpl::visitLoadInst(LoadInst &LI) {
   if (Instruction *Res = combineLoadToOperationType(*this, LI))
     return Res;
 
-  // Attempt to improve the alignment.
-  Align KnownAlign = getOrEnforceKnownAlignment(
-      Op, DL.getPrefTypeAlign(LI.getType()), DL, &LI, &AC, &DT);
-  if (KnownAlign > LI.getAlign())
-    LI.setAlignment(KnownAlign);
+  if (!EnableInferAlignmentPass) {
+    // Attempt to improve the alignment.
+    Align KnownAlign = getOrEnforceKnownAlignment(
+        Op, DL.getPrefTypeAlign(LI.getType()), DL, &LI, &AC, &DT);
+    if (KnownAlign > LI.getAlign())
+      LI.setAlignment(KnownAlign);
+  }
 
   // Replace GEP indices if possible.
   if (Instruction *NewGEPI = replaceGEPIdxWithZero(*this, Op, LI))
@@ -1445,11 +1449,13 @@ Instruction *InstCombinerImpl::visitStoreInst(StoreInst &SI) {
   if (combineStoreToValueType(*this, SI))
     return eraseInstFromFunction(SI);
 
-  // Attempt to improve the alignment.
-  const Align KnownAlign = getOrEnforceKnownAlignment(
-      Ptr, DL.getPrefTypeAlign(Val->getType()), DL, &SI, &AC, &DT);
-  if (KnownAlign > SI.getAlign())
-    SI.setAlignment(KnownAlign);
+  if (!EnableInferAlignmentPass) {
+    // Attempt to improve the alignment.
+    const Align KnownAlign = getOrEnforceKnownAlignment(
+        Ptr, DL.getPrefTypeAlign(Val->getType()), DL, &SI, &AC, &DT);
+    if (KnownAlign > SI.getAlign())
+      SI.setAlignment(KnownAlign);
+  }
 
   // Try to canonicalize the stored type.
   if (unpackStoreToAggregate(*this, SI))