[AArch64][SVE] Ensure redundant PTEST are removed with an 'invalid' PTRUE

When a PTRUE of non-element size is encountered, the PTEST optimization
logic bails out since it cannot handle that type of PTRUE. Instead, it
should be treated as a generic predicate to allow later optimizations trigger.

Differential Revision: https://reviews.llvm.org/D138116