Updates to theory preprocess equality (#5776)

Updates to theory preprocess equality (#5776)

This makes 3 changes related to arithmetic preprocessing of equalities which revert to the original behavior of master before a129c57.

For background, the commit a129c57 unintentionally changed the default behavior slightly in 3 ways (each corrected in this PR), which led a performance regression on QF_LIA in current master.

The 3 fixes are:
(1) Rewrite equalities should be applied as a post-rewrite, not a pre-rewrite in the theory-rewrite-eq preprocessing pass. This is particularly important for equalities between ITE terms that contain other equalities recursively.
(2) theory-rewrite-eq should apply after rewriting and just before the normal theory preprocessing.
(3) The arith-brab test should call ppRewrite on the arithmetic equality it introduces, as it has a choice of whether to eliminate the equality before the lemma is sent out.