[AArch64][SVE] Optimize ptrue predicate pattern with known sve register width.

For vectors that are exactly equal to getMaxSVEVectorSizeInBits, just use
AArch64SVEPredPattern::all, which can enable the use of unpredicated ptrue when available.

TestPlan: check-llvm

Differential Revision: https://reviews.llvm.org/D108706