fortran: [PR126777] Fix ICE for IMAGE_INDEX with a TEAM argument

trans_image_index translated the scalar TEAM/TEAM_NUMBER argument with
gfc_conv_expr_descriptor, which routes rank-0 expressions to
walk_coarray and asserts there.  Use gfc_conv_expr_val, as
trans_num_images and trans_this_image already do.

PR fortran/126777

Assisted-by: Claude Opus 5

gcc/fortran/ChangeLog:

	PR fortran/126777
	* trans-intrinsic.cc (trans_image_index): Translate the team
	argument with gfc_conv_expr_val.
	(trans_num_images): Force the team_number lvalue into the
	argument block, not the enclosing one.

gcc/testsuite/ChangeLog:

	PR fortran/126777
	* gfortran.dg/coarray/image_index_team_1.f90: New test.