[llvm] [InstCombine] Do not use operand info in `replaceInInstruction` (PR #99492)

[Nikita Popov via llvm-commits]

================
@@ -4713,3 +4713,24 @@ define i8 @select_knownbits_simplify_missing_noundef(i8 %x)  {
   %res = select i1 %cmp, i8 %and, i8 0
   ret i8 %res
 }
+
+ at arr = global [2 x i32] zeroinitializer, align 4
+ at cst = constant ptr getelementptr (i8, ptr @arr, i64 4)
+
+define i32 @pr99436() {
+; CHECK-LABEL: @pr99436(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[CMP:%.*]] = icmp eq ptr getelementptr (i8, ptr @arr, i64 4), null
+; CHECK-NEXT:    [[VAL:%.*]] = load i32, ptr getelementptr (i8, ptr @arr, i64 4), align 4
+; CHECK-NEXT:    [[RET:%.*]] = select i1 [[CMP]], i32 [[VAL]], i32 0
+; CHECK-NEXT:    ret i32 [[RET]]
+;
+entry:
+  %alloc = alloca ptr, align 8
+  call void @llvm.memcpy.p0.p0.i64(ptr align 8 %alloc, ptr align 8 @cst, i64 8, i1 false)
----------------
nikic wrote:

How about this?
```llvm
@g = external global i8

define i32 @test(ptr align 4 dereferenceable(4) %ptr) {
  %cmp = icmp eq ptr %ptr, @g
  %val = load i32, ptr %ptr, align 4
  %ret = select i1 %cmp, i32 %val, i32 0
  ret i32 %ret
}
```

It's not quite as nice as the replacement with null, but `@g` is still potentially non-derefable.

https://github.com/llvm/llvm-project/pull/99492