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

[Sam Elliott via llvm-commits]

https://github.com/lenary approved this pull request.

LGTM, and does seem to be neater, for these commutable cases.

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