tree-optimization/104511 - avoid FP to DFP conversion for VEC_PACK_TRUNC

tree-optimization/104511 - avoid FP to DFP conversion for VEC_PACK_TRUNC

This avoids forwprop from matching DFP <-> FP vector conversions
using VEC_[UN]PACK{_TRUNC,_LO,_HI}.  Maybe DFP vectors shouldn't be
a thing, but they appearantly are.  Re-using CONVERT/NOP_EXPR for
DFP <-> FP conversions was probably a mistake.

2022-02-14  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/104511
	* tree-ssa-forwprop.cc (simplify_vector_constructor): Avoid
	touching DFP <-> FP conversions.

	* gcc.dg/pr104511.c: New testcase.