[testsuite] require -Ofast for vect-ifcvt-18 even without avx

The test expects transformations that depend on -Ofast on x86*, but
that option is only passed when the avx_runtime is available.

Split -Ofast out of the avx conditional, so that it is passed on the
same targets that expect the transformation.


for  gcc/testsuite/ChangeLog

	* gcc.dg/vect/vect-ifcvt-18.c: Split -Ofast out of
	avx_runtime.