[ConstantFold] Don't try to strip fp -> int bitcasts to simplify icmps

ConstantFold has logic to take icmp (bitcast x to y), null and strip the
bitcast. This makes sense in general, but not if x has floating-point type. In
this case, we'd need a fcmp, not an icmp, and the code will assert. We normally
don't see this situation because we constant fold fp -> int bitcasts, however,
we'll see it for bitcasts of ppc_fp128 -> i128. This is because that bitcast is
Endian-dependent, and as a result, we don't simplify it in ConstantFold (we
could, but no one has yet added the necessary logic). Regardless, ConstantFold
should not depend on that canonicalization for correctness.

llvm-svn: 268534