Reduce scope of a few 'class loop *loop' variables

Reduce scope of a few 'class loop *loop' variables

Further clean-up after commit e41ba804ba5f5ca433e09238d561b1b4c8b10985
"Use range-based for loops for traversing loops".  No functional change.

	gcc/
	* cfgloop.c (verify_loop_structure): Reduce scope of
	'class loop *loop' variable.
	* ipa-fnsummary.c (analyze_function_body): Likewise.
	* loop-init.c (fix_loop_structure): Likewise.
	* loop-invariant.c (calculate_loop_reg_pressure): Likewise.
	* predict.c (predict_loops): Likewise.
	* tree-loop-distribution.c (loop_distribution::execute): Likewise.
	* tree-vectorizer.c (pass_vectorize::execute): Likewise.