nvc0/ir: remove merge/split pairs to allow normal propagation to occur

Because the TGSI interface creates merges for each instruction source
and then splits them back out, there are a lot of unnecessary
merge/split pairs which do essentially nothing. The various modifier/etc
propagation doesn't know how to walk though those, so just remove them
when they're unnecessary.

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