InstCombine: simplify comparisons to zero of (shl %x, Cst) or (mul %x, Cst)

This simplification happens at 2 places :
 - using the nsw attribute when the shl / mul is used by a sign test
 - when the shl / mul is compared for (in)equality to zero

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@177856 91177308-0d34-0410-b5e6-96231b3b80d8