nvc0/ir: only try to check for zero LOD if we aren't already forcing it

nvc0/ir: only try to check for zero LOD if we aren't already forcing it

There's a levelZero flag which forces texturing to pick level zero (and
not consume an explicit LOD argument). This is set for MS targets, but
could also be set for any other incoming instruction. As that is what
determines whether a LOD argument is present, check that rather than the
more indirect isMS logic.

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