Convert LiveRegUnits methods to the current convention (it's new code).
Remove extra indentation in LiveRegUnits.
Move LiveRegUnits implementation into .cpp. Comment and format.
Use a SparseSet in LiveRegUnits.

Some clients may add block live ins and may track liveness over a
large scope. This guarantees an efficient implementation in all cases
with no memory allocation/deallocation, independent of the number of
target registers. It could be slightly less convenient but is fine in
the expected case.
LiveRegUnits::removeRegsInMask safety.

Clobbering is exclusive not inclusive on register units.
For liveness, we need to consider all the preserved registers.
e.g. A regmask that clobbers YMM0 may preserve XMM0.
Units are only clobbered when all super-registers are clobbered.