nv50/ir: use unordered_set instead of list to keep track of var uses

nv50/ir: use unordered_set instead of list to keep track of var uses

The set of variable uses does not need to be ordered in any way, and
removing/adding elements is a fairly common operation in various
optimization passes.

This shortens runtime of piglit test fp-long-alu to ~22s from ~4h

Signed-off-by: Tobias Klausmann <tobias.johannes.klausmann@mni.thm.de>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>