[AArch64] Eliminate Common SUBS by Reassociating Non-Constants (#123344)
Commit 1eed46960c217f9480865702f06fb730c7521e61 added logic to
reassociate a (add (add x y) -c) operand to a CSEL instruction with a
comparison involving x and c (or a similar constant) in order to obtain
a common (SUBS x c) instruction.
This commit extends this logic to non-constants. In this way, we also
reassociate a (sub (add x y) z) operand of a CSEL instruction to
(add (sub x z) y) if the CSEL compares x and z, for example.
Alive proof: https://alive2.llvm.org/ce/z/SEVpR
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