[llvm] [RISCV] Merge Xqci Decoder Tables (PR #128140)

[Sam Elliott via llvm-commits]

lenary wrote:

> @lenary do you want to land this first and I'll rebase #128102

No. please land yours first. I intend to rebase over the Xqcilia patch and work out if we're going to have encoding difficulties when trying to fit instructions into minor gaps in the encoding space. 

https://github.com/llvm/llvm-project/pull/128140