i965/fs: Make half(fs_reg, unsigned) handle register files more explicitly
i965/fs: Make half(fs_reg, unsigned) handle register files more explicitly

Previously, we had a special case for uniforms and immediates and then a
bunch of asserts for various other pessimal things.  This commit changes it
so that it explicitly does something on each register file.  Some of them
are disallowed and others are treated properly.

Reviewed-by: Kenneth Graunke <bd2562f754ec92342f93f61c25d731e290a2ffa8@whitecape.org>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler