[LLVM][SVE] Improve legalisation of fixed length get.active.lane.mask (#90213)

We are effectively performing type and operation legalisation very early
within the code generation flow. This results in worse code quality
because the DAG is not in canonical form, which DAGCombiner corrects
through the introduction of operations that are not legal.

This patchs splits and moves the code to where type and operation
legalisation is typically implemented.