A couple of bootstrap issues on some targets:

68346: My earlier change to avoid folding the arguments to 
warn_tautological_cmp wasn't quite right, either.  This patch folds 
within the function, at the place where we are interested in a constant 
value.

68361: The way we were trying to suppress -Wparentheses before wasn't 
effective enough.  Let's actually turn off the flag around the relevant 
convert call.

Tested x86_64-pc-linux-gnu, applying to trunk.