[x86] add folds for FP logic with vector zeros

The 'or' case shows up in copysign. The copysign code also had 
redundant checking for a scalar zero operand with 'and', so I 
removed that. 

I'm not sure how to test vector 'and', 'andn', and 'xor' yet, 
but it seems better to just include all of the logic ops since
we're fixing 'or' anyway.

llvm-svn: 282546