These tables were indexed by [register][subreg index] which made them,
very large and sparse.
Replace them with lists of sub-register indexes that match the existing
lists of sub-registers. MCRI::getSubReg() becomes a very short linear
search, like getSubRegIndex() already was.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@160843 91177308-0d34-0410-b5e6-96231b3b80d8