Each execution of an alloca creates a new allocation, distinct from the
allocations of all other executions. They must not be merged, either by
CSE or via isUniformAcrossVFsAndUFs.
Update those to explicitly handle Alloca recipes.
Fixes https://github.com/llvm/llvm-project/issues/229389.
PR: https://github.com/llvm/llvm-project/pull/229929
(cherry picked from commit 95ea23b7d5b06c60c513c575577d5b7d0ed15721)