[RISCV] Move some test cases from rv64i-single-softfloat.ll to a new rv64i-double-softfloat.ll. NFC

This new test covers both with and without the F extension enabled.

This shows that the fptosi/fptoui for double->i32 use a different
libcall depending on whether the F extension is enabled. If it's
not enabled we use the 'si' library call. If it is enabled we use 'di'.