[MC] Restore a special case to support limited A-B folding when A/B are in the...

[MC] Restore a special case to support limited A-B folding when A/B are in the same fragment being laided out

Add subsection-if.s to test what we can fold (in the same fragment) and what we cannot.

Fix https://github.com/ClangBuiltLinux/linux/issues/1876
Fixes: 4bdc7f7a