[CodeGen] Simplify updateLiveIn in MachineSink (#79831)

When a whole register is added a basic block's liveins, use
LaneBitmask::getAll for the live lanes instead of trying to calculate an
accurate mask of the lanes that comprise the register.

This simplifies the code and matches other places where a whole register
is marked as livein.

This also avoids problems when regunits that are synthesized by TableGen
to represent ad hoc aliasing have a lane mask of 0.

Fixes #78942