Transform: "icmp eq (trunc (lshr(X, cst1)), cst" to "icmp (and X, mask), cst"

when X has multiple uses.  This is useful for exposing secondary optimizations,
but the X86 backend isn't ready for this when X has a single use.  For example,
this can disable load folding.

This is inching towards resolving PR6627.

llvm-svn: 130238