x86/process: Optimize TIF checks in __switch_to_xtra()

x86/process: Optimize TIF checks in __switch_to_xtra()

Help the compiler to avoid reevaluating the thread flags for each checked
bit by reordering the bit checks and providing an explicit xor for
evaluation.

With default defconfigs for each arch,

x86_64: arch/x86/kernel/process.o
text       data     bss     dec     hex
3056       8577      16   11649    2d81	Before
3024	   8577      16	  11617	   2d61	After

i386: arch/x86/kernel/process.o
text       data     bss     dec     hex
2957	   8673	      8	  11638	   2d76	Before
2925	   8673       8	  11606	   2d56	After

Originally-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Kyle Huey <khuey@kylehuey.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Andy Lutomirski <luto@kernel.org>
Link: http://lkml.kernel.org/r/20170214081104.9244-2-khuey@kylehuey.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>