[llvm] SplitKit: Take register class directly from instruction definition (PR #129727)

[Mikael Holmén via llvm-commits]

mikaelholmen wrote:

> I've verified that this patch solves downstream problems I bisected to #122110 which I mentioned in [#122110 (comment)](https://github.com/llvm/llvm-project/pull/122110#issuecomment-2696651713)

As I mentioned in
 https://github.com/llvm/llvm-project/pull/122110#issuecomment-2705509544 
after more downstream testing I see that while many problems we saw was fixed with this patch, there are still
verifier errors like
```
*** Bad machine code: Illegal virtual register for instruction ***
- function:    func_4
- basic block: %bb.32 cont299 (0x557fac4ba7e0) [6672B;7040B)
- instruction: 6680B	%1145:anh_rn = ldx_or_optionally_mv_nimm16_a16 0, 0, $noreg, 0
- operand 0:   %1145:anh_rn
Expected a aNh_0_7 register, but got a aNh_rN register
```
And this happens for our downstream target so I can't share a reproducer. A bit problematic. :/

https://github.com/llvm/llvm-project/pull/129727