fortran: Fix up initializers of param(0) PARAMETERs [PR103691]

fortran: Fix up initializers of param(0) PARAMETERs [PR103691]

On the gfortran.dg/pr103691.f90 testcase the Fortran ICE emits
  static real(kind=4) a[0] = {[0 ... -1]=2.0e+0};
That is an invalid RANGE_EXPR where the maximum is smaller than the minimum.

The following patch fixes that.  If TYPE_MAX_VALUE is smaller than
TYPE_MIN_VALUE, the array is empty and so doesn't need any initializer,
if the two are equal, we don't need to bother with a RANGE_EXPR and
can just use that INTEGER_CST as the index and finally for the 2+ values
in the range it uses a RANGE_EXPR as before.

2022-03-25  Jakub Jelinek  <jakub@redhat.com>

	PR fortran/103691
	* trans-array.cc (gfc_conv_array_initializer): If TYPE_MAX_VALUE is
	smaller than TYPE_MIN_VALUE (i.e. empty array), ignore the
	initializer; if TYPE_MIN_VALUE is equal to TYPE_MAX_VALUE, use just
	the TYPE_MIN_VALUE as index instead of RANGE_EXPR.