tree-optimization/109046 - re-combine complex loads

The following addresses PR109046 by adding an optimization to forwprop
to combine a piecewise complex load to a complex load when there are
no other uses of the components.  That's something useful in general and
easier to do than avoiding the splitting in complex lowering.

The testcase exercises both the manual and the complex lowering case.

	PR tree-optimization/109046
	* tree-ssa-forwprop.cc (pass_forwprop::execute): Combine
	piecewise complex loads.

	* gcc.dg/tree-ssa/forwprop-39.c: New testcase.