[DAGCombiner] Add combine for (not (strict_fsetcc)) to create a strict_fsetcc with the opposite condition.
Unlike the existing code that I modified here, I only handle the
case where the strict_fsetcc has a single use. Not sure exactly
how to handle multiples uses.
Testing this on X86 is hard because we already have a other
combines that get rid of lowered version of the integer setcc that
this xor will eventually become. So this combine really just
saves a bunch of extra nodes being created. Not sure about other
Differential Revision: https://reviews.llvm.org/D71816