The change fixes a bug in the SROA where tree-structured merge
optimization was incorrectly applied when the size of the stored value
was not a multiple of the new allocated element type size. The original
change is https://github.com/llvm/llvm-project/pull/152793. A simple
repro would be
```
define <1 x i32> @foo(<1 x i16> %a, <1 x i16> %b) {
entry:
%alloca = alloca [1 x i32]
%ptr0 = getelementptr inbounds [2 x i16], ptr %alloca, i32 0, i32 0
store <1 x i16> %a, ptr %ptr0
%ptr1 = getelementptr inbounds [2 x i16], ptr %alloca, i32 0, i32 1
store <1 x i16> %b, ptr %ptr1
%result = load <1 x i32>, ptr %alloca
ret <1 x i32> %result
}
```
Currently, this will lead to a compile time crash.
In this change, we will skip the tree-structured merge for this case and
fall back to normal SROA.
8faeed042a