This patch adds lane tracking to SI Scheduler.
To handle lanes,
. When a register is always used with all its lanes, it replaces the register with RegisterMaskPair(Reg, LaneBitMask::all())
. In the other cases, it determines a 'basis' of masks such that any register/lane usage can be decomposed into fake registers of RegisterMaskPair(Reg, LaneBitMask element of the basis).
Previously the code assumed that a Register cannot be defined if already defined. LaneMasks break this assumption.
Decomposing into unique RegisterMaskPair "registers" (such that the Lanes don't intersect) enables to reuse the previous assumption.
A RegisterMaskPair can only be defined if not already alive. Thus enables to use the previous code, with some updates to how register usage is computed.