[SystemZ] Use NILL instruction instead of NILF where possible

Summary: SystemZ shift instructions only use the last 6 bits of the shift
amount. When the result of an AND operation is used as a shift amount, this
means that we can use the NILL instruction (which operates on the last 16 bits)
rather than NILF (which operates on the last 32 bits) for a 16-bit savings in
instruction size.

Reviewers: uweigand

Subscribers: llvm-commits

Author: colpell
Committing on behalf of Elliot.

Differential Revision: http://reviews.llvm.org/D21686

llvm-svn: 274066