[AArch64] Avoid selecting XZR inline ASM memory operand

Restricting register class to PointerRegClass for memory operands.

Also fix the PointerRegClass for AArch64 from GPR64 to GPR64sp, since
XZR cannot hold a memory pointer while SP is.

Fixes PR33134.

Differential Revision: https://reviews.llvm.org/D34999

llvm-svn: 308060