[InstCombine] Fold multiuse shr eq zero

The single-use case is handled implicity by converting the icmp
into a mask check first. When comparing with zero in particular,
we don't need the one-use restriction, as we only produce a single
icmp.

https://alive2.llvm.org/ce/z/MSixcm
https://alive2.llvm.org/ce/z/GwpG0M