nv50/ir: make a copy of tex src if it's referenced multiple times

nv50/ir: make a copy of tex src if it's referenced multiple times

For nv50 we coalesce the srcs and defs into a single node. As such, we
can end up with impossible constraints if the source is referenced
after the tex operation (which, due to the coalescing of values, will
have overwritten it).

This logic already exists for inserting moves for MERGE/UNION sources.
It's the exact same idea here, so leverage that code, which also
includes a few optimizations around not extending live ranges
unnecessarily.

Fixes tests/spec/glsl-1.30/execution/fs-textureSize-components.shader_test

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>