middle-end/112622 - convert and vector-to-float

middle-end/112622 - convert and vector-to-float

The following avoids ICEing when trying to convert a vector to
a scalar float.

	PR middle-end/112622
	* convert.cc (convert_to_real_1): Use element_precision
	where a vector type might appear.  Provide specific
	diagnostic for unexpected vector argument.

	* gcc.dg/pr112622.c: New testcase.
	* gcc.dg/simd-2.c: Adjust.
	* gcc.target/i386/vect-bfloat16-typecheck_1.c: Likewise.
	* gcc.target/i386/vect-bfloat16-typecheck_2.c: Likewise.