i386: Increase Skylake SImode pseudo register store cost

i386: Increase Skylake SImode pseudo register store cost

On Skylake, SImode store cost isn't less than half cost of 128-bit vector
store.  This patch increases Skylake SImode pseudo register store cost to
make it the same as QImode and HImode.

gcc/

	PR target/91446
	* config/i386/x86-tune-costs.h (skylake_cost): Increase SImode
	pseudo register store cost from 3 to 6 to make it the same as
	QImode and HImode.

gcc/testsuite/

	PR target/91446
	* gcc.target/i386/pr91446.c: New test.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@275905 138bc75d-0d04-0410-961f-82ee72b054a4