Define mask as extern instead of uninitialized local variables.

Define mask as extern instead of uninitialized local variables.

The testcases are supposed to scan for vpopcnt{b,w,d,q} operations
with k mask, but mask is defined as uninitialized local variable which
will be set as 0 at rtl expand phase.
And it's further simplified off by late_combine which caused scan assembly failure.
Move the definition of mask outside to make the testcases more stable.

gcc/testsuite/ChangeLog:

	PR target/115610
	* gcc.target/i386/avx512bitalg-vpopcntb.c: Define mask as
	extern instead of uninitialized local variables.
	* gcc.target/i386/avx512bitalg-vpopcntbvl.c: Ditto.
	* gcc.target/i386/avx512bitalg-vpopcntw.c: Ditto.
	* gcc.target/i386/avx512bitalg-vpopcntwvl.c: Ditto.
	* gcc.target/i386/avx512vpopcntdq-vpopcntd.c: Ditto.
	* gcc.target/i386/avx512vpopcntdq-vpopcntq.c: Ditto.