tree-optimization/105250 - adjust fold_convertible_p PR105140 fix

tree-optimization/105250 - adjust fold_convertible_p PR105140 fix

The following reverts the original PR105140 fix and goes for instead
applying the additional fold_convert constraint for VECTOR_TYPE
conversions also to fold_convertible_p.  I did not try sanitizing
all of this at this point.

2022-04-13  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/105250
	* fold-const.cc (fold_convertible_p): Revert
	r12-7979-geaaf77dd85c333, instead check for size equality
	of the vector types involved.

	* gcc.dg/pr105250.c: New testcase.