[InstCombine] add tests to show missed vector folds; NFC

The shift-shift possibilities became easier to see after:
https://reviews.llvm.org/rL292145



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@292150 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed