tree-optimization/107686 - fix bitfield ref through vec_unpack optimization

The following propely restricts the bitfield access to integral types
when we look through VEC_UNPACK with the intent to emit a widening
conversion.

	PR tree-optimization/107686
	* tree-ssa-forwprop.cc (optimize_vector_load): Restrict
	VEC_UNPACK support to integral typed bitfield refs.

	* gcc.dg/pr107686.c: New testcase.