[InstSimplify] fix folds for (0.0 - X) + X --> 0 (PR27151)

As shown in:
https://bugs.llvm.org/show_bug.cgi?id=27151
...the existing fold could miscompile when X is NaN.

The fold was also dependent on 'ninf' but that's not necessary.

From IEEE-754 (with default rounding which we can assume for these opcodes):
"When the sum of two operands with opposite signs (or the difference of two 
operands with like signs) is exactly zero, the sign of that sum (or difference) 
shall be +0...However, x + x = x − (−x) retains the same sign as x even when 
x is zero."

llvm-svn: 327575