TargetLowering: Improve handling of (setcc ([sz]ext x) 0, cc) in SimplifySetCC

Summary:
When SimplifySetCC sees a setcc node that compares the result of a
value extension operation with a constant, it tries to simplify the
setcc node by eliminating the extension and shrinking the constant.

If shrinking the inputs to setcc is deemed not desirable by the target
(e.g. the target does not want a setcc comparing i1 values), then it
is still possible to optimize this sequence in some cases.

This patch adds the following combines to SimplifySetCC when shrinking setcc
inputs is not desirable:

(setcc ([sz]ext (setcc x, y, cc)), 0, setne) -> (setcc (x, y, cc))
(setcc ([sz]ext (setcc x, y, cc)), 0, seteq) -> (setcc (x, Y, !cc))

There are no tests for this yet, but once AMDGPU correctly implements
TargetLowering::isTypeDesirableForOp(), this new combine will be
exercised by the existing CodeGen/AMDGPU/setcc-opt.ll test.

Reviewers: resistor, arsenm

Subscribers: jroelofs, arsenm, llvm-commits

Differential Revision: http://reviews.llvm.org/D15034

llvm-svn: 258067