The longobject.c warnings are almost certainly unrelated to the test_re crash.

If shifting right twice (adding parens for clarity):

    (LONG_MAX >> PyLong_SHIFT) >> PyLong_SHIFT.

squashes the warnings, that would be a substantially clearer way to express the intent than the

    SIZEOF_LONG*CHAR_BIT-1 >= 2*PyLong_SHIFT

spelling.  Adding a comment *explaining* the intent would be even better.

For the segfault issue, best guess is that it's a compiler optimization bug.  Not common as mud, or even as common as nuisance warnings, but not all that rare either ;-)