[llvm] [RISCV] Support zext.h mnemonic with Zbkb. (PR #96821)

[Craig Topper via llvm-commits]

topperc wrote:

> FYI, GNU binutils as doesn't emit pack/packw for zext.h when zbkb is available:
> 
> ```
> dtcxzyw at dtcxzyw:~/WorkSpace/Projects/compilers/LLVM/llvm-build$ cat xxx.S
> zext.h t0, t1
> dtcxzyw at dtcxzyw:~/WorkSpace/Projects/compilers/LLVM/llvm-build$ riscv64-linux-gnu-gcc -march=rv64gc_zbkb xxx.S -c
> dtcxzyw at dtcxzyw:~/WorkSpace/Projects/compilers/LLVM/llvm-build$ riscv64-linux-gnu-objdump xxx.o -S
> 
> xxx.o:     file format elf64-littleriscv
> 
> 
> Disassembly of section .text:
> 
> 0000000000000000 <.text>:
>    0:   03031293                slli    t0,t1,0x30
>    4:   0302d293                srli    t0,t0,0x30
> ```

I know. I'm going to try to get that changed too.

https://github.com/llvm/llvm-project/pull/96821