[InstCombine] icmp Pred (shl nsw X, C1), C0 --> icmp Pred X, C0 >> C1

Try harder to fold icmp with shl nsw as discussed here:
http://lists.llvm.org/pipermail/llvm-dev/2017-January/108749.html

This is similar to the 'shl nuw' transforms that were added with D25913.

This may eventually help solve:
https://llvm.org/bugs/show_bug.cgi?id=30773

Differential Revision: https://reviews.llvm.org/D28406

llvm-svn: 292492