[llvm] [TableGen] Fix inferring missing sub-classes for various subreg indices (PR #185638)

[Mikael Holmen via llvm-commits]

mikaelholmen wrote:

Yes, this fixes the "Error: attempt to dereference a past-the-end iterator." problem I saw when compiling with EXPENSIVE_CHECKS, which I reported in https://github.com/llvm/llvm-project/pull/183371#issuecomment-4002642328

Thanks!

https://github.com/llvm/llvm-project/pull/185638