Fortran: reject bad SIZE argument while simplifying ISHFTC [PR106911]

gcc/fortran/ChangeLog:

	PR fortran/106911
	* simplify.cc (gfc_simplify_ishftc): If the SIZE argument is known
	to be outside the allowed range, terminate simplification.

gcc/testsuite/ChangeLog:

	PR fortran/106911
	* gfortran.dg/pr106911.f90: New test.