Follow up to e4284a7c70cd "[AMDGPU] 4-align SGPR triples".
Previously TTMP triples like ttmp[3:5] were aligned on a 3-TTMP boundary
which has no basis in hardware.
Aligning them on a 4-TTMP boundary matches what we do for SGPRs, which
reduces the number of extra register classes synthesized by TableGen,
bringing the total number down from 653 to 615.
02ed65912e