glsl: Rename linker's is_varying_var
glsl: Rename linker's is_varying_var

Both the ast->IR and linker have functions with this name, but different
behavior.

Rename the linker's version to var_counts_against_varying_limit to be
closer to what it is actually used for.

Suggested by Ian a while back.

Signed-off-by: Chris Forbes <5dcac483e7a6e31a5cbe89f38cc69dff8ee93dac@ijw.co.nz>
Reviewed-by: Anuj Phogat <e2026475996c48f0b97fd37e84129b3912980428@gmail.com>

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