[AMDGPU] Fix copying a machine operand

Without this I get:

*** Bad machine code: Instruction has operand with wrong parent set ***
- function:    available_externally_test
- basic block: %bb.0  (0x7dad598)
- instruction: %0:r600_treg32_x = MOV 1, 0, 0, 0, $alu_literal_x, 0, 0, 0, -1, 1, $pred_sel_off, @available_externally, 0

Differential Revision: https://reviews.llvm.org/D111549