[llvm] [GISel][RISCV] Legalize shifts with non-trivial shamt types (PR #93019)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> For legalizer changes, MIR tests are preferable.

This was mostly useful for bringup and edge case handling, but overall MIR tests are harder to maintain and can miss the system not working as a whole 

https://github.com/llvm/llvm-project/pull/93019