Fix a thinko: When lowering fneg with xor, bitcast the operands

Fix a thinko: When lowering fneg with xor, bitcast the operands
from floating-point to integer first, and bitcast the result
back to floating-point. Previously, this test was passing by
falling back to SelectionDAG lowering. The resulting code isn't
as nice, but it's correct and CodeGen now stays on the fast path.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@81171 91177308-0d34-0410-b5e6-96231b3b80d8