[InstCombine] add unsigned saturation subtraction canonicalizations

This is the instcombine part of unsigned saturation canonicalization.
Backend patches already commited: 
https://reviews.llvm.org/D37510
https://reviews.llvm.org/D37534

It converts unsigned saturated subtraction patterns to forms recognized 
by the backend:
(a > b) ? a - b : 0 -> ((a > b) ? a : b) - b)
(b < a) ? a - b : 0 -> ((a > b) ? a : b) - b)
(b > a) ? 0 : a - b -> ((a > b) ? a : b) - b)
(a < b) ? 0 : a - b -> ((a > b) ? a : b) - b)
((a > b) ? b - a : 0) -> - ((a > b) ? a : b) - b)
((b < a) ? b - a : 0) -> - ((a > b) ? a : b) - b)
((b > a) ? 0 : b - a) -> - ((a > b) ? a : b) - b)
((a < b) ? 0 : b - a) -> - ((a > b) ? a : b) - b)

Patch by Yulia Koval!

Differential Revision: https://reviews.llvm.org/D41480

llvm-svn: 324255