[Codegen][X86][AArch64][ARM][PowerPC] Inc-of-add vs sub-of-not (PR42457)

Summary:
This is the backend part of [[ https://bugs.llvm.org/show_bug.cgi?id=42457 | PR42457 ]].
In middle-end, we'd want to prefer the form with two adds - D63992,
but as this diff shows, not every target will prefer that pattern.

Out of 4 targets for which i added tests all seem to be ok with inc-of-add for scalars,
but only X86 prefer that same pattern for vectors.

Here i'm adding a new TLI hook, always defaulting to the inc-of-add,
but adding AArch64,ARM,PowerPC overrides to prefer inc-of-add only for scalars.

Reviewers: spatel, RKSimon, efriedma, t.p.northover, hfinkel

Reviewed By: efriedma

Subscribers: nemanjai, javed.absar, kristof.beyls, kbarton, jsji, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64090

llvm-svn: 365010