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

[Thorsten Schütt via llvm-commits]

tschuett wrote:

One observation: If you create G_X in the lowering pass, you have to regbank-select them, but we have a pass which is an expert for it. In this case, it is probably better to do the lowering in the legalizer. As a by-product, we get legal G_X.

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