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.