[AArch64][AsmParser] Fix bug in operand printer

The switch in AArch64Operand::print was changed in D45688 so the shift
can be printed after printing the register. This is implemented with
LLVM_FALLTHROUGH and was broken in D52485 when BTIHint was put between
the register and shift operands.

Reviewed By: ostannard

Differential Revision: https://reviews.llvm.org/D86535