[ARM][MVE] Tail-predication: clean-up of unused code
After the rewrite of this pass (D79175) I missed one thing: the inserted VCTP
intrinsic can be cloned to exit blocks if there are instructions present in it
that perform the same operation, but this wasn't triggering anymore. However,
it turns out that for handling reductions, see D75533, it's actually easier not
not to have the VCTP in exit blocks, so this removes that code.
This was possible because it turned out that some other code that depended on
this, rematerialization of the trip count enabling more dead code removal
later, wasn't doing much anymore due to more aggressive dead code removal that
was added to the low-overhead loops pass.
Differential Revision: https://reviews.llvm.org/D82773