PR/20742
* Makefile.in (tree-chrec.o): Depend on params.h.
* params.def (PARAM_SCEV_MAX_EXPR_SIZE): New parameter with
default value 20.
* tree-chrec.c: Depend on params.h. Replace build with buildN,
and fold build with fold_buildN.
(chrec_fold_plus_1): Fail with a chrec_don_know when the size of
the expression exceeds PARAM_SCEV_MAX_EXPR_SIZE.
(tree_contains_chrecs): Compute an estimation of the size of the
given expression.
* tree-chrec.h (tree_contains_chrecs): Modify its declaration.
(tree_does_not_contain_chrecs): Update the use of tree_contains_chrecs.
* tree-scalar-evolution.c (simple_iv): Ditto.
* doc/invoke.texi (scev-max-expr-size): Documented.