[VPlan] Don't apply predication discount to non-originally-predicated blocks (#160449)

Split off from #158690. Currently if an instruction needs predicated due
to tail folding, it will also have a predicated discount applied to it
in multiple places.
This is likely inaccurate because we can expect a tail folded
instruction to be executed on every iteration bar the last.

This fixes it by checking if the instruction/block was originally
predicated, and in doing so prevents vectorization with tail folding
where we would have had to scalarize the memory op anyway.

On llvm-test-suite this causes 4 loops in total to no longer be
vectorized with -O3 on arm64-apple-darwin, and there's no observable
performance impact.