Simplify gimple_assign_load

The following simplifies and outlines gimple_assign_load.  In
particular it is not necessary to get at the base of the possibly
loaded expression but just handle the case of a single handled
component wrapping a non-memory operand.

	* gimple.h (gimple_assign_load): Outline...
	* gimple.cc (gimple_assign_load): ... here.  Avoid
	get_base_address and instead just strip the outermost
	handled component, treating a remaining handled component
	as load.