[SROA] Reject illegal vector memset promotion (#216772)
SROA can select vector promotion for an `alloca` partition containing a
memset and vector accesses whose element type is an unstable pointer.
We discovered this while building Julia with LLVM 21 and assertions
enabled. SROA crashed in `convertValue` because `canConvertValue`
rejected converting an integer splat to a non-integral pointer:
```
Assertion `canConvertValue(DL, OldTy, NewTy, VScale) &&
"Value not convertable to type"' failed.
```
Current main no longer asserts because `convertValue` was replaced by
`CreateBitPreservingCastChain`. However, the underlying unsupported
transformation remains.
For example:
```llvm
target datalayout = "e-ni:10"
declare void @llvm.memset.p0.i64(ptr, i8, i64, i1)
define <2 x ptr addrspace(10)> @f(
<2 x ptr addrspace(10)> %v, i8 %byte) {
%a = alloca [2 x ptr addrspace(10)], align 8
store <2 x ptr addrspace(10)> %v, ptr %a, align 8
%p = getelementptr inbounds i8, ptr %a, i64 8
call void @llvm.memset.p0.i64(ptr %p, i8 %byte, i64 8, i1 false)
%r = load <2 x ptr addrspace(10)>, ptr %a, align 8
ret <2 x ptr addrspace(10)> %r
}
```
Running `opt -passes=sroa` on current main introduces:
```llvm
%zext = zext i8 %byte to i64
%isplat = mul i64 %zext, 72340172838076673
%ptr = inttoptr i64 %isplat to ptr addrspace(10)
```
The legacy `ni:10` data-layout entry marks address space 10 as having an
unstable pointer representation. AFAIU, optimizers may not introduce new
dynamic `inttoptr` operations for such pointers.
Fix: Check the integer-splat conversion during vector-promotion
viability and reject the promotion when it cannot be converted to the
vector element type. This leaves the original memory operations intact.
Signed-off-by: Tim Besard <tim.besard@gmail.com>
