https://crrev.com/c/2072858 rewrote the implementation of non-unicode
ignore-case matches to comply with the JS spec in some corner
cases. It fixed character matches and character class matches.

We missed a similar bug in the implementation of back references. This
CL fixes that bug.

The main change is in regexp-macro-assembler.cc, where
CaseInsensitiveCompareUC16 is split into CaseInsensitiveCompareUnicode
(which has the same semantics as before) and
CaseInsensitiveCompareNonUnicode (which has the semantics described
here: https://tc39.es/ecma262/#sec-runtime-semantics-canonicalize-ch).

Most of the rest of the patch undoes https://crrev.com/c/2081816 to
once again make the unicode flag available to the macroassembler, so
that we can decide which helper function to call.

The testcase is a version of test/intl/regress-10248.js, modified to
test backreferences.

Bug: v8:10573
Change-Id: I70ef7d134d37f99b1f75a5eba17020e82d59f1b9
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2219284Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68129}