[InstCombine] refactor foldICmpShlConstant(); NFCI

This reduces the size of and increases the symmetry with the planned functional change in:
https://reviews.llvm.org/D28406


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@292260 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed