Split compare instruction handling OUT of handleTwoArgFP into handleCompareFP.

This makes the code much simpler, and the two cases really do belong apart.
Once we do it, it's pretty obvious how flawed the logic was for A != A case,
so I fixed it (fixing PR369).

This also uses freeStackSlotAfter instead of inserting an fxchg then
popStackAfter'ing in the case where there is a dead result (unlikely, but
possible), producing better code.

llvm-svn: 14139