tree-optimization/117417 - ICE with complex load optimization

tree-optimization/117417 - ICE with complex load optimization

When we decompose a complex load only used as real and imaginary
parts we fail to honor IL constraints which are that a BIT_FIELD_REF
of register type should be outermost in a ref.  The following
simply avoids the transform when the complex load has such a
BIT_FIELD_REF.

	PR tree-optimization/117417
	* tree-ssa-forwprop.cc (pass_forwprop::execute): Avoid
	decomposing BIT_FIELD_REF complex load.

	* gcc.dg/torture/pr117417.c: New testcase.