tree-optimization/98211 - fix bogus vectorization of conversion

tree-optimization/98211 - fix bogus vectorization of conversion

Pattern recog incompletely handles some bool cases but we shouldn't
miscompile as a result but not vectorize.  Unfortunately
vectorizable_assignment lets invalid conversions (that
vectorizable_conversion rejects) slip through.  The following
rectifies that.

2020-12-10  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/98211
	* tree-vect-stmts.c (vectorizable_assignment): Disallow
	invalid conversions to bool vector types.

	* gcc.dg/pr98211.c: New testcase.