[AArch64] Fix checks for AArch64MCExpr::VK_SABS flag.

VK_SABS is part of the SymLoc bitfield in the variant kind which should
be compared for equality, not by checking the VK_SABS bit.

As far as I know, the existing code happened to produce the correct
results in all cases, so this is just a cleanup.

Patch by Stephen Crane.

Differential Revision: https://reviews.llvm.org/D60596

llvm-svn: 358788