[ARM] Introduce t2WhileLoopStartTP

This adds t2WhileLoopStartTP, similar to the t2DoLoopStartTP added in
D90591. It keeps a reference to both the tripcount register and the
element count register, so that the ARMLowOverheadLoops pass in the
backend can pick the correct one without having to search for it from
the operand of a VCTP.

Differential Revision: https://reviews.llvm.org/D103236