[LV] get.active.lane.mask consuming tripcount instead of backedge-taken count

This adapts LV to the new semantics of get.active.lane.mask as discussed in
D86147, which means that the LV now emits intrinsic get.active.lane.mask with
the loop tripcount instead of the backedge-taken count as its second argument.
The motivation for this is described in D86147.

Differential Revision: https://reviews.llvm.org/D86304