[CodeGen] Add scalable vector support for lowering of llvm.get.active.lane.mask

Currently the generic lowering of llvm.get.active.lane.mask is done
in SelectionDAGBuilder::visitIntrinsicCall and currently assumes
only fixed-width vectors are used. This patch changes the code to be
more generic and support scalable vectors too. I have added tests
for SVE here:

  CodeGen/AArch64/active_lane_mask.ll

although the code quality leaves a lot to be desired. The code will
be improved significantly in a later patch that makes use of the
SVE whilelo instruction.

Differential Revision: https://reviews.llvm.org/D114541