[llvm] [AArch64] Share materialized constant between add x, k and sub x,k (PR #225430)

[Paul Walker via llvm-commits]

================

----------------
paulwalker-arm wrote:

Given this is block level, is it worth adding a visitor specifically for MOVI to populate the cache with available constants? Then change visitADDSUB to query the cache looking for a negated version of their immediate operand.

https://github.com/llvm/llvm-project/pull/225430