bitint: Make sure BEXTC checks extension when optimized

bitint: Make sure BEXTC checks extension when optimized

In BEXTC, whether a _BitInt object is properly extended is examined
by a value comparison against a copied object in a wider _BitInt
type that utilizes all of the partial limb.

Since the (implicit) conversion to the wider type may be optimized
away now and cause the result of the comparison to always be true,
we need to cast the copied object down to the original type to
force a extension, so that it can serve as our reference.

	* gcc.dg/bitintext.h (BEXTC1): Define.  Convert the copied
	object back to the original type before comparison.
	(BEXTC): Use BEXTC1 for both the signed and the unsigned case.