extend cache_integer_cst

extend cache_integer_cst

This modules-related patch extends cache_integer_cst.  Currently, when
given a small cst, that cst is added to the type's small and /must
not/ already be there.  Large values are fine if they are already in
the large cache.  This adds a parameter to indicate small duplicates
are ok, and it returns the cached value -- either what was already
tehre, or the newly inserted const.

	gcc/
	* tree.h (cache_integer_cst): Add defaulted might_duplicate parm.
	* tree.c (cache_integer_cst): Return the integer cst, add
	might_duplicate parm to permit finding a small duplicate.