[x86] Handle more cases where we can re-use an atomic operation's flags

rather than doing a separate comparison.

This both saves an explicit comparision and avoids the use of `xadd`
which introduces register constraints and other challenges to the
generated code.

The motivating case is from atomic reference counts where `1` is the
sentinel rather than `0` for whatever reason. This can and should be
lowered efficiently on x86 by just using a different flag, however the
x86 code only handled the `0` case.

There remains some further opportunities here that are currently hidden
due to canonicalization. I've included test cases that show these and
FIXMEs. However, I don't at the moment have any production use cases and
they seem substantially harder to address.

Differential Revision: https://reviews.llvm.org/D36945

llvm-svn: 311317