LoopVectorize: Clean up ValueMap a bit and avoid double lookups.

No intended functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173809 91177308-0d34-0410-b5e6-96231b3b80d8
Fixing warnings revealed by gcc release build
    
Fixed set-but-not-used warnings.

Reviewer: gribozavr


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@173810 91177308-0d34-0410-b5e6-96231b3b80d8