freedreno/ir3: eliminate unused false-deps

freedreno/ir3: eliminate unused false-deps

Previously false-dependencies would get flagged as used, even if the
only "use" was a false dep to (for example) prevent a load from being
scheduled after a store.

In addition to being pointless instructions, in some cases they can
cause problems.  For example, ldg (and similar instructions) depend on
an immed arg getting CP'd into the instruction, but this doesn't happen
if an instruction is otherwise unused.  Which can result in undefined
results (overwriting unintended registers).

Signed-off-by: Rob Clark <robdclark@gmail.com>