[llvm] [RISCV] Remove unneeded zext.h instructions (PR #225960)

[via llvm-commits]

bbbill42 wrote:

@topperc I moved the SignedMatch check to a helper function and covered the logic for trunc correctly, this seems to work correctly on my benchmark, if this code looks ok I'll have AI write up some testcases and start getting ready for a PR under codegen? I can try to have the PR come from a repo that's aligned with my public account.

Apparently it still  has a bug in letting ops like shl through, trying to clear that, this is a rabbit hole, but surely digging deeper will fix the problem.

https://github.com/llvm/llvm-project/pull/225960