[InstCombine] Fold a shifty implementation of clamp-to-allones.

Summary:
Fold
or(ashr(subNSW(Y, X), ScalarSizeInBits(Y)-1), X)
into
X s> Y ? -1 : X

https://rise4fun.com/Alive/d8Ab

clamp255 is a common operator in image processing, can be implemented
in a shifty way "(255 - X) >> 31 | X & 255". Fold shift into select
enables more optimization, e.g., vmin generation for ARM target.

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

Reviewed By: lebedev.ri

Subscribers: kristof.beyls, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D67800

llvm-svn: 372678