[ARM][MVE] Tail-predication: remove the BTC + 1 overflow checks

This adapts tail-predication to the new semantics of get.active.lane.mask as
defined in D86147. This means that:
- we can remove the BTC + 1 overflow checks because now the loop tripcount is
  passed in to the intrinsic,
- we can immediately use that value to setup a counter for the number of
  elements processed by the loop and don't need to materialize BTC + 1.

Differential Revision: https://reviews.llvm.org/D86303