[AMDGPU] Kill def when folding immediate in two-addr pass

Two-address pass works right before RA and if an immediate
was folded into an instruction there is nothing to remove
the dead def. We end up with something like:

	v_mov_b32_e32 v14, 0xc1700000
	v_mov_b32_e32 v14, 0x41200000
	v_fmaak_f32 v51, s67, v19, 0xc1700000
	v_fmaak_f32 v38, v51, v19, 0x4120000

The patch kills the dead move instruction right in the folding.

Differential Revision: https://reviews.llvm.org/D114999