ira: Add ira_subloop_allocnos_can_differ_p

ira: Add ira_subloop_allocnos_can_differ_p

color_pass has two instances of the same code for propagating non-cap
assignments from parent loops to subloops.  This patch adds a helper
function for testing when such propagations are required for correctness
and uses it to remove the duplicated code.

A later patch will use this in ira-build.c too, which is why the
function is exported to ira-int.h.

No functional change intended.

gcc/
	PR rtl-optimization/98782
	* ira-int.h (ira_subloop_allocnos_can_differ_p): New function,
	extracted from...
	* ira-color.c (color_pass): ...here.