i965/vec4: Don't coalesce registers with overlapping writes not matching the MOV source.
i965/vec4: Don't coalesce registers with overlapping writes not matching the MOV source.

In preparation for adding support for sub-GRF offsets to the VEC4 IR.

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

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler