[AArch64] Check opcode before trying to extract register from operand

When matching FNEG patterns for the MachineCombiner we need to check for
opcodes first, before trying to extract a register from an operand.
Otherwise handling of instructions with non-register operands causes the
compiler to crash.

Differential Revision: https://reviews.llvm.org/D158473

(cherry picked from commit 979e8ae4)