[mips] Define register class FGRH32 for the high half of the 64-bit floating

[mips] Define register class FGRH32 for the high half of the 64-bit floating
point registers. We will need this register class later when we add
definitions for instructions mfhc1 and mthc1. Also, remove sub-register indices
sub_fpeven and sub_fpodd and use sub_lo and sub_hi instead.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188842 91177308-0d34-0410-b5e6-96231b3b80d8