[InstCombine] Add additional memcpy from constant test with phi (NFC)

This is the case that is safe to handle, but currently isn't.

Author: nikic

llvm/test/Transforms/InstCombine/ptr-replace-alloca.ll

@@ -281,6 +281,35 @@ join:
   ret i32 %v
 }
 
+define i32 @addrspace_diff_remove_alloca(i1 %cond) {
+; CHECK-LABEL: @addrspace_diff_remove_alloca(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[A:%.*]] = alloca [32 x i8], align 1
+; CHECK-NEXT:    call void @llvm.memcpy.p0.p1.i64(ptr noundef nonnull align 1 dereferenceable(32) [[A]], ptr addrspace(1) noundef align 16 dereferenceable(32) @g2, i64 32, i1 false)
+; CHECK-NEXT:    [[GEP:%.*]] = getelementptr inbounds [32 x i8], ptr [[A]], i64 0, i64 2
+; CHECK-NEXT:    br i1 [[COND:%.*]], label [[IF:%.*]], label [[JOIN:%.*]]
+; CHECK:       if:
+; CHECK-NEXT:    br label [[JOIN]]
+; CHECK:       join:
+; CHECK-NEXT:    [[PHI:%.*]] = phi ptr [ [[A]], [[IF]] ], [ [[GEP]], [[ENTRY:%.*]] ]
+; CHECK-NEXT:    [[V:%.*]] = load i32, ptr [[PHI]], align 4
+; CHECK-NEXT:    ret i32 [[V]]
+;
+entry:
+  %a = alloca [32 x i8]
+  call void @llvm.memcpy.p0.p1.i64(ptr %a, ptr addrspace(1) @g2, i64 32, i1 false)
+  %gep = getelementptr inbounds [32 x i8], ptr %a, i32 0, i32 2
+  br i1 %cond, label %if, label %join
+
+if:
+  br label %join
+
+join:
+  %phi = phi ptr [ %a, %if ], [ %gep, %entry ]
+  %v = load i32, ptr %phi
+  ret i32 %v
+}
+
 define i32 @phi_loop(i1 %c) {
 ; CHECK-LABEL: @phi_loop(
 ; CHECK-NEXT:  entry: