glsl: Clean up apply_implicit_conversion

glsl: Clean up apply_implicit_conversion

We're about to add new implicit conversions, first for ARB_gpu_shader5,
and then later for ARB_gpu_shader_fp64. Pull out the opcode
determination into its own function, and get rid of the bool -> float
case that could never be hit anyway [since it fails the is_numeric()
check].

V2: Retain the vector width mangling. It turns out this is necessary for
the conversions done (and then thrown away) when determining the return
type of arithmetic operators.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>