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

[via llvm-commits]

bbbill42 wrote:

@joelkevinjones - this now changes generic TypePromotion rather than just the RISC-V backend. Could you take a look from the AArch64 side?

It helped mcf on the JH7110 by about 4%, and gave me 11% on my 8 wide ooo design, but it was originally a machine only merge, @topperc pointed me at TypePromotion and things escalated from there.

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