[RISCV] Merge GPRPair and GPRF64Pair (#116094)

As suggested by Craig, this tries to merge the two sets of register
classes created in #112983, GPRPair* and GPRF64Pair*.

- I added some explicit annotations to `RISCVInstrInfoD.td` which fixed
the type inference issues I was seeing from tablegen for select
patterns.
- I've had to make the behaviour of `splitValueIntoRegisterParts` and
`joinRegisterPartsIntoValue` cover more cases, because you cannot
bitcast to/from untyped (the bitcast would otherwise have been inserted
automatically by TargetLowering code).
- I apparently didn't need to change `getNumRegisters` again, which
continues to tell me there's a bug in the code for tied inputs. I added
some more test coverage of this case but it didn't seem to help find the
asserts I was finding before - I think the difference is between the
default behaviour for integers which doesn't apply to floats.
- There's still a difference between BuildGPRPair and BuildPairF64 (and
the same for SplitGPRPair and SplitF64). I'm not happy with this, I
think it's quite confusing, as they're very similar, just differing in
whether they give a `untyped` or a `f64`. I haven't really worked out
how the DAGCombiner copes if one meets the other, I know we have some of
this for the f64 variants already, but they're a lot more complex than
the GPRPair variants anyway.