[AMDGPU] Handle memset users in PromoteAlloca

Allows allocas with memset users to be promoted.

This is intended to prevent patterns such as `memset(&alloca, 0, sizeof(alloca))` (which I think can be emitted by frontends) from preventing a vectorization of allocas.

Fixes SWDEV-388784

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D146225