[llvm] [RISCV][GISEL] Introduce the RISCVPostLegalizerLowering pass (PR #108991)

[Michael Maitland via llvm-commits]

michaelmaitland wrote:

> Are you sure you need this?

We took this because we wanted to avoid introducing target specific generic opcodes in the legalizer so that the post legalization combiner would only have to consider non-target specific generic opcodes. We still need to lower before instruction selection to target specific generic opcodes. We thought that this was the place to do it. It looks like thats what AArch64 does.

In order to understand whether we need this, I'd first like to ask two questions:
1. Why does AArch64 take the post legalizer lowering approach instead of lowering to generic instructions in the selector
2. Why isn't this pass marked deprecated if it is not needed? (Or is it marked deprecated and did I miss that?)


https://github.com/llvm/llvm-project/pull/108991