glsl: Allow dot() on scalars, and throw out dotlike().

glsl: Allow dot() on scalars, and throw out dotlike().

In all uses of dotlike() we're writing generic code that operates on 1-4
component vectors. That our IR requires ir_binop_dot expressions'
operands to be 2+ component vectors is an implementation detail that's
not important when implementing built-in functions with dot(), which is
defined for scalar floats in GLSL.

Reviewed-by: Eric Anholt <eric@anholt.net>