re PR middle-end/53153 (ice in tree_low_cst, at tree.c:6569)

gcc/
	PR middle-end/53153
	* gimplify.c (preprocess_case_label_vec_for_gimple): New function,
	split out from ...
	(gimplify_switch_expr): ... here.
	* gimple.h (preprocess_case_label_vec_for_gimple): Add prototype.
	* tree-ssa-forwprop.c (simplify_gimple_switch_label_vec): New function
	to clean up case labels with values outside the index type range.
	(simplify_gimple_switch): Call it if something changed.
	Remove strange and unnecessary assert.

testsuite/
	PR middle-end/53153
	* gcc.dg/pr53153.c: New test.

From-SVN: r187048