[InstructionSimplify] Add support for saturating add/sub

Add support for saturating add/sub in InstructionSimplify. In particular, the following simplifications are supported:

    sat(X + 0) -> X
    sat(X + undef) -> -1
    sat(X uadd MAX) -> MAX
    (and commutative variants)

    sat(X - 0) -> X
    sat(X - X) -> 0
    sat(X - undef) -> 0
    sat(undef - X) -> 0
    sat(0 usub X) -> 0
    sat(X usub MAX) -> 0

Patch by: @nikic (Nikita Popov)

Differential Revision: https://reviews.llvm.org/D54532

llvm-svn: 347330