c-family: Fix up shorten_compare for decimal vs. non-decimal float comparison [PR104510]

c-family: Fix up shorten_compare for decimal vs. non-decimal float comparison [PR104510]

The comment in shorten_compare says:
  /* If either arg is decimal float and the other is float, fail.  */
but the callers of shorten_compare don't expect anything like failure
as a possibility from the function, callers require that the function
promotes the operands to the same type, whether the original selected
*restype_ptr one or some shortened.
So, if we choose not to shorten, we should still promote to the original
*restype_ptr.

2022-02-16  Jakub Jelinek  <jakub@redhat.com>

	PR c/104510
	* c-common.cc (shorten_compare): Convert original arguments to
	the original *restype_ptr when mixing binary and decimal float.

	* gcc.dg/dfp/pr104510.c: New test.