intel/compiler: Explicitly cast register type in switch

intel/compiler: Explicitly cast register type in switch

brw_reg::type is "enum brw_reg_type type:4".  For whatever reason, GCC
is treating this as an int instead of an enum.  As a result, it doesn't
detect missing switch cases and it doesn't detect that flow can get out
of the switch.

This silences the warning:

src/intel/compiler/brw_reg.h: In function ‘bool brw_regs_negative_equal(const brw_reg*, const brw_reg*)’:
src/intel/compiler/brw_reg.h:305:1: warning: control reaches end of non-void function [-Wreturn-type]
 }
 ^

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Brian Paul <brianp@vmware.com>