Add BasicTTIImpl cost model for llvm.get.active.lane.mask intrinsic

The vectoriser sometimes generates predicated vector loops using
the llvm.get.active.lane.mask intrinsic so it's important that we
are able to calculate a valid cost for the call instruction. When
SVE is enabled we are able to use a single whilelo instruction
for some vector types - in such cases I've marked the cost as 1.
For all other cases I've set the cost according to how the intrinsic
will be expanded.

Tests added here:

  Analysis/CostModel/AArch64/sve-intrinsics.ll
  Analysis/CostModel/ARM/active_lane_mask.ll
  Analysis/CostModel/RISCV/active_lane_mask.ll

Differential Revision: https://reviews.llvm.org/D121109