* lisp/emacs-lisp/pcase.el (pcase--if): Try to invert test to reduce depth.

* lisp/emacs-lisp/pcase.el (pcase--if): Try to invert test to reduce depth.
(pcase-mutually-exclusive-predicates): New var.
(pcase--split-consp, pcase--split-pred): Use it.
(pcase--split-equal, pcase--split-member): When splitting against
a pure predicate, run it to know the outcome.
(pcase--u1): Mark vars that are actually used.
(pcase--q1): Avoid introducing unused vars.