[Hexagon] Separate Hexagon subreg indices for different register classes

For pairs of 32-bit registers: isub_lo, isub_hi.
For pairs of vector registers: vsub_lo, vsub_hi.

Add generic subreg indices: ps_sub_lo, ps_sub_hi, and a function
  HexagonRegisterInfo::getHexagonSubRegIndex(RegClass, GenericSubreg)
that returns the appropriate subreg index for RegClass.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@286377 91177308-0d34-0410-b5e6-96231b3b80d8
25 files changed