[llvm] [AArch64] isTBLMask(M, VT) as part of the shuffle mask check (PR #79058)

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

The approach here looks reasonable; assuming we actually emit appropriate TBL ops, marking them cheap seems appropriate.  May need some work to deal with indirect effects of this. Needs testcases.

https://github.com/llvm/llvm-project/pull/79058