Teach InstCombine to nuke a common alloca pattern -- an alloca which has

Teach InstCombine to nuke a common alloca pattern -- an alloca which has
GEPs, bit casts, and stores reaching it but no other instructions. These
often show up during the iterative processing of the inliner, SROA, and
DCE. Once we hit this point, we can completely remove the alloca. These
were actually showing up in the final, fully optimized code in a bunch
of inliner tests I've been working on, and notably they show up after
LLVM finishes optimizing away all function calls involved in
hash_combine(a, b).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@154285 91177308-0d34-0410-b5e6-96231b3b80d8