Stop only if there aren't any usable algorithms

Stop only if there aren't any usable algorithms

When searching for an usable algorithm with -minline-all-stringops,
decide_alg stops when it sees libcall even if there is a usable
algorithm.  It goes into an infinite loop.  This patch changes
decide_alg to stop searching only if there aren't any usable algorithms.
Testd on Linux/x86-64.

gcc/

	PR target/64108
	* config/i386/i386.c (decide_alg): Stop only if there aren't
	any usable algorithms.

gcc/testsuite/

	PR target/64108
	* gcc.target/i386/memset-strategy-2.c: New test.

From-SVN: r218272