[DAGCombiner][X86][AArch64][AMDGPU] (x + C) - y -> (x - y) + C fold

Summary:
The main motivation is shown by all these `neg` instructions that are now created.
In particular, the `@reg32_lshr_by_negated_unfolded_sub_b` test.

AArch64 test changes all look good (`neg` created), or neutral.

X86 changes look neutral (vectors), or good (`neg` / `xor eax, eax` created).

I'm not sure about `X86/ragreedy-hoist-spill.ll`, it looks like the spill
is now hoisted into preheader (which should still be good?),
2 4-byte reloads become 1 8-byte reload, and are elsewhere,
but i'm not sure how that affects that loop.

I'm unable to interpret AMDGPU change, looks neutral-ish?

This is hopefully a step towards solving [[ https://bugs.llvm.org/show_bug.cgi?id=41952 | PR41952 ]].

https://rise4fun.com/Alive/pkdq (we are missing more patterns, i'll submit them later)

Reviewers: craig.topper, RKSimon, spatel, arsenm

Reviewed By: RKSimon

Subscribers: bjope, qcolombet, kzhuravl, jvesely, wdng, nhaehnle, yaxunl, javed.absar, dstuttard, tpr, t-tye, kristof.beyls, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62223

llvm-svn: 361852

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