[ARM][MVE] Tail Predicate IsSafeToRemove

Introduce a method to walk through use-def chains to decide whether
it's possible to remove a given instruction and its users. These
instructions are then stored in a set until the end of the transform
when they're erased. This is now used to perform checks on the
iteration count (LoopDec chain), element count (VCTP chain) and the
possibly redundant iteration count.

As well as being able to remove chains of instructions, we know also
check that the sub feeding the vctp is producing the expected value.

Differential Revision: https://reviews.llvm.org/D71837