explow: Allow dynamic allocations after vregs

explow: Allow dynamic allocations after vregs

This patch allows allocate_dynamic_stack_space to be called before
or after virtual registers have been instantiated.  It uses the
same approach as allocate_stack_local, which already supported this.

gcc/
	* function.h (get_stack_dynamic_offset): Declare.
	* function.cc (get_stack_dynamic_offset): New function,
	split out from...
	(get_stack_dynamic_offset): ...here.
	* explow.cc (allocate_dynamic_stack_space): Handle calls made
	after virtual registers have been instantiated.