This is split off from D79100 and adds a new target hook emitGetActiveLaneMask
that can be queried to check if the intrinsic @llvm.get.active.lane.mask() is
supported by the backend and if it should be emitted for a given loop.

See also commit rG7fb8a40e and its commit message for more details/context
on this new intrinsic.

Differential Revision: https://reviews.llvm.org/D80597