[AArch64][SVE] Fix BRKNS bug in optimizePTestInstr

The BRKNS instruction is unlike the other instructions that set flags
since it has an all active implicit predicate, so the existing

  PTEST(PG, BRKN(PG, A, B)) -> BRKNS(PG, A, B)

in AArch64InstrInfo::optimizePTestInstr is incorrect, however

  PTEST(PTRUE_B(31), BRKN(PG, A, B)) -> BRKNS(PG, A, B)

is correct.

Spotted by @paulwalker-arm in D134946.

Reviewed By: paulwalker-arm

Differential Revision: https://reviews.llvm.org/D135655