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

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

https://rise4fun.com/Alive/lFH

Fold shift into select enables more optimization,
e.g., vmax generation for ARM target.

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

Reviewed By: lebedev.ri

Subscribers: xbolva00, andreadb, craig.topper, RKSimon, kristof.beyls, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D67799

llvm-svn: 372676