[llvm] [RISCV][MC] Add aliases for beq/bne with x0 as the first argument => beqz/bnez (PR #139086)

[Alex Bradbury via llvm-commits]

asb wrote:

Although this received several LGTMs, Craig raised the point about divergence between llvm-objdump and GNU objdump. While I'm not sure that identical output for llvm-objdump is necessarily the utmost priority, I do agree that practically speaking doing this canonicalisation in simplifyInstruction handles the cases that bothered me. I've posted #141781 to do that.

https://github.com/llvm/llvm-project/pull/139086