remove almost all users of gimple_expr_code

remove almost all users of gimple_expr_code

This replaces the old-school gimple_expr_code with more selective
functions throughout the compiler, in all cases making the code
shorter or more clear.

2020-11-13  Richard Biener  <rguenther@suse.de>

	* cfgexpand.c (gimple_assign_rhs_to_tree): Use
	gimple_assign_rhs_class.
	(expand_gimple_stmt_1): Likewise.
	* gimplify-me.c (gimple_regimplify_operands): Use
	gimple_assign_single_p.
	* ipa-icf-gimple.c (func_checker::compare_gimple_assign):
	Remove redundant compare.
	(func_checker::compare_gimple_cond): Use gimple_cond_code.
	* tree-ssa-tail-merge.c (gimple_equal_p): Likewise.
	* predict.c (predict_loops): Use gimple_assign_rhs_code.