[SelectionDAG] Legalize intrinsic get.active.lane.mask

This adapts legalization of intrinsic get.active.lane.mask to the new semantics
as described in D86147. Because the second argument is now the loop tripcount,
we legalize this intrinsic to an 'icmp ULT' instead of an ULE when it was the
backedge-taken count.

Differential Revision: https://reviews.llvm.org/D86302