Remove superfluous non-operands from LDXR, STXR, and related instructions on AArch64

On AArch64 we have superfluous operands in the exclusive load and store instructions in DR's IR:

  0x0000ffff7d459b24  885f7c01   ldxr   (%x0)[4byte] $0x1f $0x1f -> %w1
  0x0000ffff7d459af0  885ffc01   ldaxr  (%x0)[4byte] $0x1f $0x1f -> %w1

  0x0000ffff7d459afc  88027c04   stxr   %w4 $0x1f -> (%x0)[4byte] %w2
  0x0000ffff7d459b28  8803fc02   stlxr  %w2 $0x1f -> (%x0)[4byte] %w3

These 0x1f integers seem to be just part of the encoding and aren't actually operands. I would expect them to not show up in the IR at all.