[InstCombine] use m_APInt to allow icmp (shl X, Y), C folds for splat constant vectors, part 1

This is a partial enablement (move the ConstantInt guard down) because there are many
different folds here and one of the later ones will require reworking 'isSignBitCheck'.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@279339 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed