[Backport] ifcvt: Don't lower bitfields with non-constant offsets [PR 111882]

[Richard Ball]

Hi,

Requesting permission to backport:
https://gcc.gnu.org/git/gitweb.cgi?p=gcc.git;h=24cf1f600b8ad34c68a51f48884e72d01f729893
to gcc-13 in order to fix:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111882

Applies cleanly and with no regressions.

Thanks,
Richard