[InstCombine] fold signed absolute diff patterns

This overlaps partially with the codegen patch D144789. This needs no-wrap
for correctness, and I'm not sure if there's an unsigned equivalent:
https://alive2.llvm.org/ce/z/ErmQ-9
https://alive2.llvm.org/ce/z/mr-c_A

This is obviously an improvement in IR, and it looks like a codegen win
for all targets and data types that I sampled.

The 'nabs' case is left as a potential follow-up (and seems less likely
to occur in real code).

Differential Revision: https://reviews.llvm.org/D145073

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