[InstCombine] Add test cases for various add/subtracts of constants(scalar, splat, and vector) with phis and selects. Improvements coming in a future commit.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@299476 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed