[AArch64] [XRay] Account for XRay event instrs in Branch Relaxation

PATCHABLE_TYPED_EVENT_CALL and PATCHABLE_EVENT_CALL are pseudo
instructions that expand to XRay sleds, so getInstSizeInBytes
should reflect the size of the sleds, not the pseudo-instructions.

Differential Revision: https://reviews.llvm.org/D156272