[CGP] In optimizeMemoryInst handle select similarly to phi

[CGP] In optimizeMemoryInst handle select similarly to phi

This lets us optimize away selects that perform the same address computation in
two different ways and is also the first step towards being able to handle
selects between two different, but compatible, address computations.

Differential Revision: https://reviews.llvm.org/D38242

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@314794 91177308-0d34-0410-b5e6-96231b3b80d8