forwprop: Fix ICE when building an identity constructor [PR94784]

forwprop: Fix ICE when building an identity constructor [PR94784]

In the testcase for PR94784, we have two vectors with the same ABI identity
but with different TYPE_MODEs. It would be better to flip the assert around
so that it checks that the two vectors have equal TYPE_VECTOR_SUBPARTS and
that converting the corresponding element types is a useless_type_conversion_p.

2020-04-27  Felix Yang  <felix.yang@huawei.com>

gcc/
	PR tree-optimization/94784
	* tree-ssa-forwprop.c (simplify_vector_constructor): Flip the
	assert around so that it checks that the two vectors have equal
	TYPE_VECTOR_SUBPARTS and that converting the corresponding element
	types is a useless_type_conversion_p.

gcc/testsuite/
	PR tree-optimization/94784
	* gcc.dg/pr94784.c: New test.