[X86] Use SADDSAT/SSUBSAT instead of ADDS/SUBS

Migrate the X86 backend from X86ISD opcodes ADDS and SUBS to generic
ISD opcodes SADDSAT and SSUBSAT. This also improves scodegen for
@llvm.sadd.sat() and @llvm.ssub.sat() intrinsics.

This is a followup to D55787 and part of PR40056.

Differential Revision: https://reviews.llvm.org/D55833

llvm-svn: 349520