i965/fs: Remove force_writemask_all assertion for execsize < 8.
i965/fs: Remove force_writemask_all assertion for execsize < 8.

This doesn't seem to be necessary.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=86974
Reviewed-by: Kenneth Graunke <bd2562f754ec92342f93f61c25d731e290a2ffa8@whitecape.org>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler