[llvm] [RISCV] Add guard to prevent GPRPair merge on targets without Zdinx or P (PR #186600)

[Kavin Gnanapandithan via llvm-commits]

KavinTheG wrote:

> Right now this is sort-of enforced with a bunch of bools and invoking very similar copies of functions, maybe that's where the confusion is coming from?

I'm not sure what's the best way to solve, 
> maybe we're not merging cm.mv* pairs where the a1 move comes before the a0 move

Apologies if I'm misunderstanding your statement, but I think it is able to merge moves where a1 comes before the a0 move according to the comments in `mergePairedInsns`? It swaps the order so that instruction with `a0`/`X10` comes first.  

```
  //   mv a1, s1
  //   mv a0, s2    =>  cm.mva01s s2,s1
  //
  //   mv a0, s2
  //   mv a1, s1    =>  cm.mva01s s2,s1
```


https://github.com/llvm/llvm-project/pull/186600