nv50/ir: make sure to fetch all sources before creating instruction

nv50/ir: make sure to fetch all sources before creating instruction

We must fetch all sources into the instruction stream before generating
the instruction that uses them. Otherwise we'll define values after
using them, which won't work so well.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Tested-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>