[llvm] [AMDGPU] Enable i8 GEP promotion for vector allocas (PR #166132)

[Shilei Tian via llvm-commits]

shiltian wrote:

> Could you please give me an example?

In the test case, the `%index` is for `%alloca` of `<3 x float>`, which is 12-byte, each of which has 4-byte. Since the GEP is of type `i8`, what if `%index` is 5, which will be somewhere in the middle of the 2nd element of `%alloca`.

https://github.com/llvm/llvm-project/pull/166132