[ARM][MVE] tail-predication: overflow checks for elementcount, cont'd

This is a reimplementation of the overflow checks for the elementcount,
i.e. the 2nd argument of intrinsic get.active.lane.mask. The element
count is lowered in each iteration of the tail-predicated loop, and
we must prove that this expression doesn't overflow.

Many thanks to Eli Friedman and Sam Parker for all their help with
this work.

Differential Revision: https://reviews.llvm.org/D88086