i965/fs: Return more accurate read size for LINTERP from fs_inst::size_read.
i965/fs: Return more accurate read size for LINTERP from fs_inst::size_read.

The LINTERP virtual instruction only reads three scalar components
from the first 16B of the second source, we can now teach size_read()
about it since its return value is represented with byte granularity.

Reviewed-by: Iago Toral Quiroga <a847aabaac44184372f6b1cd1a59aa254daaeb0b@igalia.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler