[Intrinsic] Add sshl.sat/ushl.sat, saturated shift intrinsics.

Summary:
This patch adds two intrinsics, llvm.sshl.sat and llvm.ushl.sat,
which perform signed and unsigned saturating left shift,
respectively.

These are useful for implementing the Embedded-C fixed point
support in Clang, originally discussed in
http://lists.llvm.org/pipermail/llvm-dev/2018-August/125433.html
and
http://lists.llvm.org/pipermail/cfe-dev/2018-May/058019.html

Reviewers: leonardchan, craig.topper, bjope, jdoerfert

Subscribers: hiraditya, jdoerfert, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D83216