[CodeGen] Improve SelectionDAGBuilder lowering code for get.active.lane.mask intrinsic

Previously we were using UADDO to generate a two-result value with
the unsigned addition and the overflow mask. We then combined the
overflow mask with the trip count comparison to get a result.
However, we don't need to do this - we can simply use a UADDSAT
saturating add node to add the vector index splat and the stepvector
together. Then we can just compare this to a splat of the trip count.
This results in overall better code quality for both Thumb2 and AArch64.

Differential Revision: https://reviews.llvm.org/D115354