[AArch64][SVE] Remove redundant PTEST following PNEXT/PFIRST

PNEXT and PFIRST set the NZCV flags, so the subsequent PTEST can be
optimized away in AArch64InstrInfo::optimizePTestInstr.

See-also: https://reviews.llvm.org/D93292

Differential Revision: https://reviews.llvm.org/D110177