fold-const: Treat fp conversion to a type with same mode as copy

fold-const: Treat fp conversion to a type with same mode as copy

In function fold_convert_const_real_from_real, when the modes of
two types involved in fp conversion are the same, we can simply
take it as copy, rebuild with the exactly same TREE_REAL_CST and
the target type.  It is more efficient and helps to avoid possible
unexpected signalling bit clearing in [1].

[1] https://gcc.gnu.org/pipermail/gcc-patches/2022-December/608533.html

gcc/ChangeLog:

	* fold-const.cc (fold_convert_const_real_from_real): Treat floating
	point conversion to a type with same mode as copy instead of normal
	convertFormat.