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

[Luke Lau via llvm-commits]

lukel97 wrote:

Looks like this affects AArch64 too, adding in some reviewers for those changes. We might want to add an aarch64 test case to this PR

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