[llvm] [RISCV][GISEL] Legalize and post-legalize lower G_INSERT_SUBVECTOR (PR #108859)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> I stated before: the odd thing about the lower passes: they are free-standing functions. There is no CombinerHelper and no access to LegalizerInfo. In the combiner, we have the pattern:
> 

I don't understand the lowering passes. They seem like they're just a hack and I don't understand why they are needed.

https://github.com/llvm/llvm-project/pull/108859