SGPR_128 only includes the real allocatable SGPRs, and SReg_128 adds
the additional non-allocatable TTMP registers. There's no point in
allocating SReg_128 vregs. This shrinks the size of the classes
regalloc needs to consider, which is usually good.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@374284 91177308-0d34-0410-b5e6-96231b3b80d8