i965/fs: Rename Gen4 physical varying pull constant load opcode.

i965/fs: Rename Gen4 physical varying pull constant load opcode.

For consistency with the Gen7 variant.  I'm not doing the same to the
uniform pull constant message at this point because the non-GEN7 one
is still overloaded to be either an expression-like logical
instruction or a Gen4-specific physical send message.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>