[X86] Improve detection of unneeded shift amount masking to also handle the...

[X86] Improve detection of unneeded shift amount masking to also handle the case that the LHS has known zeroes in it

If the LHS has known zeros, the RHS immediate will have had bits removed. So call computeKnownBits to get the known zeroes so we can handle this case.

Differential Revision: https://reviews.llvm.org/D58475

llvm-svn: 354811