Neuter the optimization I implemented with r107852 and r108258 which turn some

floating point equality comparisons into integer ones with -ffast-math. The
issue is the optimization causes +0.0 != -0.0.

Now the optimization is only done when one side is known to be 0.0. The other
side's sign bit is masked off for the comparison.

rdar://10964603

llvm-svn: 151861