[ARM] Lower unsigned saturation to USAT

Summary:
Implement lower of unsigned saturation on an interval [0, k] where k + 1 is a power of two using USAT instruction in a similar way to how [~k, k] is lowered using SSAT on ARM models that supports it.

Patch by Marten Svanfeldt

Reviewers: t.p.northover, pbarrio, eastig, SjoerdMeijer, javed.absar, fhahn

Reviewed By: fhahn

Subscribers: fhahn, aemerson, javed.absar, llvm-commits, kristof.beyls

Differential Revision: https://reviews.llvm.org/D41348

llvm-svn: 321164