rtl.texi (const_double): Document as sign-extending.

	* doc/rtl.texi (const_double): Document as sign-extending.
	* expmed.c (expand_mult): Ensure we don't use shift
	incorrectly.
	* emit-rtl.c (immed_double_int_const): Refine to state the
	value is signed.
	* simplify-rtx.c (mode_signbit_p): Add a fixme for wider than
	CONST_DOUBLE integers.
	(simplify_const_unary_operation, UNSIGNED_FLOAT): Ensure no
	negative values are converted.  Fix conversions bigger than
	HOST_BITS_PER_WIDE_INT.
	(simplify_binary_operation_1): Ensure we don't use shift
	incorrectly.
	(simplify_immed_subreg): Sign-extend CONST_DOUBLEs.
	* explow.c (plus_constant_mode): Add.
	(plus_constant): Implement with plus_constant_mode.
	* rtl.h (plus_constant_mode): Add.

From-SVN: r186147