Introduce a Vreg128 class, use it in 128-bit SIMD instructions

Summary: This provides enough information for vasm-xls to choose 64-bit
or 128-bit spill slots.

Reviewed By: @jasone

Differential Revision: D1583511
Stronger vasm dead code analysis.

Summary: Now that status flags are explicit we can compute liveness of
instructions that modify flags.

Reviewed By: @jasone

Differential Revision: D1591668