[llvm] [ARM] Reject fixed-point VCVT with different registers (PR #126232)

[Alfie Richards via llvm-commits]

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

LGTM too!

I did have a memory of Tablegen instruction records being able to contain a validation function which could have been a nicer way to do this but I cant find any record of that.

https://github.com/llvm/llvm-project/pull/126232