[MemCpyOpt] Use AA to check for MustAlias between memset and memcpy

Rather than checking for simple equality, check for MustAlias, as
we do in other transforms. This catches equivalent GEPs.

Author: nikic

llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp

@@ -1134,7 +1134,7 @@ bool MemCpyOptPass::processMemCpyMemCpyDependence(MemCpyInst *M,
 bool MemCpyOptPass::processMemSetMemCpyDependence(MemCpyInst *MemCpy,
                                                   MemSetInst *MemSet) {
   // We can only transform memset/memcpy with the same destination.
-  if (MemSet->getDest() != MemCpy->getDest())
+  if (!AA->isMustAlias(MemSet->getDest(), MemCpy->getDest()))
     return false;
 
   // Check that src and dst of the memcpy aren't the same. While memcpy

llvm/test/Transforms/MemCpyOpt/memset-memcpy-redundant-memset.ll

@@ -261,7 +261,6 @@ define void @test_same_dynamic_size(i8* noalias %src, i8* noalias %dst, i64 %siz
 define void @test_must_alias_same_size(i8* noalias %src, i8* noalias %dst, i8 %c) {
 ; CHECK-LABEL: @test_must_alias_same_size(
 ; CHECK-NEXT:    [[GEP1:%.*]] = getelementptr i8, i8* [[DST:%.*]], i64 16
-; CHECK-NEXT:    call void @llvm.memset.p0i8.i64(i8* [[GEP1]], i8 [[C:%.*]], i64 16, i1 false)
 ; CHECK-NEXT:    [[GEP2:%.*]] = getelementptr i8, i8* [[DST]], i64 16
 ; CHECK-NEXT:    call void @llvm.memcpy.p0i8.p0i8.i64(i8* [[GEP2]], i8* [[SRC:%.*]], i64 16, i1 false)
 ; CHECK-NEXT:    ret void
@@ -276,9 +275,13 @@ define void @test_must_alias_same_size(i8* noalias %src, i8* noalias %dst, i8 %c
 define void @test_must_alias_different_size(i8* noalias %src, i64 %src_size, i8* noalias %dst, i64 %dst_size, i8 %c) {
 ; CHECK-LABEL: @test_must_alias_different_size(
 ; CHECK-NEXT:    [[GEP1:%.*]] = getelementptr i8, i8* [[DST:%.*]], i64 16
-; CHECK-NEXT:    call void @llvm.memset.p0i8.i64(i8* [[GEP1]], i8 [[C:%.*]], i64 [[DST_SIZE:%.*]], i1 false)
 ; CHECK-NEXT:    [[GEP2:%.*]] = getelementptr i8, i8* [[DST]], i64 16
-; CHECK-NEXT:    call void @llvm.memcpy.p0i8.p0i8.i64(i8* [[GEP2]], i8* [[SRC:%.*]], i64 [[SRC_SIZE:%.*]], i1 false)
+; CHECK-NEXT:    [[TMP1:%.*]] = icmp ule i64 [[DST_SIZE:%.*]], [[SRC_SIZE:%.*]]
+; CHECK-NEXT:    [[TMP2:%.*]] = sub i64 [[DST_SIZE]], [[SRC_SIZE]]
+; CHECK-NEXT:    [[TMP3:%.*]] = select i1 [[TMP1]], i64 0, i64 [[TMP2]]
+; CHECK-NEXT:    [[TMP4:%.*]] = getelementptr i8, i8* [[GEP2]], i64 [[SRC_SIZE]]
+; CHECK-NEXT:    call void @llvm.memset.p0i8.i64(i8* align 1 [[TMP4]], i8 [[C:%.*]], i64 [[TMP3]], i1 false)
+; CHECK-NEXT:    call void @llvm.memcpy.p0i8.p0i8.i64(i8* [[GEP2]], i8* [[SRC:%.*]], i64 [[SRC_SIZE]], i1 false)
 ; CHECK-NEXT:    ret void
 ;
   %gep1 = getelementptr i8, i8* %dst, i64 16