Concretize gimple_switch_index and gimple_switch_index_ptr

Concretize gimple_switch_index and gimple_switch_index_ptr

This corresponds to:
  [PATCH 72/89] Concretize gimple_switch_index and gimple_switch_index_ptr
  https://gcc.gnu.org/ml/gcc-patches/2014-04/msg01185.html
from the original 89-patch kit

That earlier patch was approved by Jeff:
> OK once prereqs go in and will obviously need updating for const
> changes as well.
in https://gcc.gnu.org/ml/gcc-patches/2014-05/msg00860.html

gcc/
	* gimple.h (gimple_switch_index): Require a const_gimple_switch rather
	than a plain const_gimple.
	(gimple_switch_index_ptr): Likewise.

	* gimplify-me.c (gimple_regimplify_operands): Add checked cast to
	gimple_switch within "case GIMPLE_SWITCH".
	* tree-cfgcleanup.c (cleanup_control_expr_graph): Likewise.
	* tree-ssa-ccp.c (ccp_fold): Likewise.
	* tree-ssa-dom.c (optimize_stmt): Likewise.

	* tree-ssa-ccp.c (evaluate_stmt): Add checked cast to
	gimple_switch within region guarded by check for GIMPLE_SWITCH.
	* tree-ssa-dom.c (record_edge_info): Likewise.
	(eliminate_redundant_computations): Likewise.
	* tree-ssa-loop-ivcanon.c (tree_estimate_loop_size): Likewise.
	* tree-ssa-threadedge.c (simplify_control_stmt_condition): Likewise.

	* tree-ssa-dom.c (initialize_hash_element): Replace check for
	code GIMPLE_SWITCH with a dyn_cast<gimple_switch>.
	(propagate_rhs_into_lhs): Likewise.
	* tree-ssa-propagate.c (may_propagate_copy_into_stmt): Likewise.
	(propagate_tree_value_into_stmt): Likewise.