[LoopVectorize][AArch64] Use get.active.lane.mask intrinsic when SVE is enabled

When SVE is enabled for AArch64 targets it makes more sense to use the
get.active.lane.mask intrinsic, because SVE has an exact 1-1 mapping
from the intrinsic to the 'whilelo' instruction for legal vector types.
This instruction neatly takes overflow into account as well. This patch
fixes an issue in VPInstruction::generateInstruction that assumed we are
only dealing with fixed-width vectors.

Differential Revision: https://reviews.llvm.org/D117109