[SystemZ] Use LLGT(R) instructions

[SystemZ] Use LLGT(R) instructions

This adds support for the 31-to-64-bit zero extension instructions
LLGT and LLGTR and uses them for code generation where appropriate.

Since this operation can also be performed via RISBG, we have to
update SystemZDAGToDAGISel::tryRISBGZero so that we prefer LLGT
over RISBG in case both are possible.  The patch includes some
simplification to the tryRISBGZero code; this is not intended
to cause any (further) functional change in codegen.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@286585 91177308-0d34-0410-b5e6-96231b3b80d8