[ARM][MVE] Tail-Predication: rematerialise iteration count in exit blocks

[ARM][MVE] Tail-Predication: rematerialise iteration count in exit blocks

This patch uses helper function rewriteLoopExitValues that is refactored in
D72602 to rematerialise the iteration count in exit blocks, so that we can
clean-up loop update expressions inside the hardware-loops later in
ARMLowOverheadLoops, which is necessary to get actual performance gains for
tail-predicated loops.

Differential Revision: https://reviews.llvm.org/D72714