Make option mvzeroupper independent of optimization level.

Make option mvzeroupper independent of optimization level.

pass_insert_vzeroupper is under condition

TARGET_AVX && TARGET_VZEROUPPER
&& flag_expensive_optimizations && !optimize_size

But the document of mvzeroupper doesn't mention the insertion
required -O2 and above, it may confuse users when they explicitly
use -Os -mvzeroupper.

------------
mvzeroupper
Target Mask(VZEROUPPER) Save
Generate vzeroupper instruction before a transfer of control flow out of
the function.
------------

The patch moves flag_expensive_optimizations && !optimize_size to
ix86_option_override_internal. It makes -mvzeroupper independent of
optimization level, but still keeps the behavior of architecture
tuning(emit_vzeroupper) unchanged.

gcc/ChangeLog:

	* config/i386/i386-features.c (pass_insert_vzeroupper:gate):
	Move flag_expensive_optimizations && !optimize_size to ..
	* config/i386/i386-options.c (ix86_option_override_internal):
	.. this, it makes -mvzeroupper independent of optimization
	level, but still keeps the behavior of architecture
	tuning(emit_vzeroupper) unchanged.
	(rest_of_handle_insert_vzeroupper): Remove
	flag_expensive_optimizations && !optimize_size.

gcc/testsuite/ChangeLog:

	* gcc.target/i386/avx-vzeroupper-29.c: New testcase.