[InstCombine] Signed saturation patterns

This adds an instcombine matcher for code that attempts to perform signed
saturating arithmetic by casting to a higher type. Unsigned cases are already
matched, this adds extra matches for the more complex signed cases, which
involves matching the min(max(add a b)) nodes with proper extends to ensure
legality.

Differential Revision: https://reviews.llvm.org/D68651

llvm-svn: 375505