Eliminate a couple of fields from TargetRegisterClass: SubRegClasses and SuperRegClas...

Eliminate a couple of fields from TargetRegisterClass: SubRegClasses and SuperRegClasses. These are not necessary. Also eliminate getSubRegisterRegClass and getSuperRegisterRegClass. These are slow and their results can change if register file names change. Just use TargetLowering::getRegClassFor() to get the right TargetRegisterClass instead.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@62762 91177308-0d34-0410-b5e6-96231b3b80d8