rtl-optimization/103075 - avoid ICEing on unfolded int-to-float converts

rtl-optimization/103075 - avoid ICEing on unfolded int-to-float converts

The following avoids asserting in exact_int_to_float_conversion_p that
the argument is not constant which it in fact can be with
-frounding-math and inexact int-to-float conversions.  Say so.

2021-11-04  Richard Biener  <rguenther@suse.de>

	PR rtl-optimization/103075
	* simplify-rtx.c (exact_int_to_float_conversion_p): Return
	false for a VOIDmode operand.

	* gcc.dg/pr103075.c: New testcase.