Fortran: intrinsic ISHFTC and missing optional argument SIZE [PR67277]

gcc/fortran/ChangeLog:

	PR fortran/67277
	* trans-intrinsic.cc (gfc_conv_intrinsic_ishftc): Handle optional
	dummy argument for SIZE passed to ISHFTC.  Set default value to
	BIT_SIZE(I) when missing.

gcc/testsuite/ChangeLog:

	PR fortran/67277
	* gfortran.dg/ishftc_optional_size_1.f90: New test.