[Verifier] Additional check for intrinsic get.active.lane.mask

This adapts the verifier checks for intrinsic get.active.lane.mask to the new
semantics of it as described in D86147. I.e., the second argument %n, which
corresponds to the loop tripcount, must be greater than 0 if it is a constant,
so check that.

Differential Revision: https://reviews.llvm.org/D86301