i965: Optimize reading the destination type

i965: Optimize reading the destination type

brw_hw_type_to_reg_type() needs to know only whether the file is
BRW_IMMEDIATE_VALUE or not, which is not a valid file for the
destination. gcc and clang will evaluate __builtin_strcmp() at compile
time, so we can use it to pass a constant file for the destination.

   text	   data	    bss	    dec	    hex	filename
7816214	 346248	 420496	8582958	 82f72e	i965_dri.so before
7816070	 346248	 420496	8582814	 82f69e	i965_dri.so after

Reviewed-by: Scott D Phillips <scott.d.phillips@intel.com>