re PR rtl-optimization/8178 (__builtin_ffs broken with -march=k6)

	PR optimization/8178
	* config/i386/i386.md (*movsi_zero): Delete.
	(*ffs_no_cmove): Use ix86_expand_clear to zero the third operand.

Co-Authored-By: Richard Henderson <rth@redhat.com>

From-SVN: r72680