[InstCombine] remove dead code/tests; NFCI

These patterns and tests were added to InstSimplify with:
https://reviews.llvm.org/rL303004


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@308096 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed