[NFC][InstCombine] Add tests for shifty implementation of clamping.

Summary:
Clamp negative to zero and clamp positive to allOnes are common
operation in image saturation.

Add tests for shifty implementation of clamping, as prepare work for
folding:

and(ashr(subNSW(Y, X), ScalarSizeInBits(Y)-1), X) --> X s> 0 ? X : 0;

or(ashr(subNSW(Y, X), ScalarSizeInBits(Y)-1), X) --> X s> Y ? allOnes : X.

Reviewers: lebedev.ri, efriedma, spatel, kparzysz, bcahoon

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D67798

llvm-svn: 372671