SelectionDAG: Clean up LegalizeSetCCCondCode() function

Interpreting the results of this function is not very intuitive, so I
cleaned it up to make it more clear whether or not a SETCC op was
legalized and how it was legalized (either by swapping LHS and RHS or
replacing with AND/OR).

This patch does change functionality in the LHS and RHS swapping case,
but unfortunately there are no in-tree tests for this.  However, this
patch is a prerequisite for R600 to take advantage of the LHS and RHS
swapping, so tests will be added in subsequent commits.
SelectionDAG: Try to expand all condition codes using getCCSwappedOperands()

This is useful for targets like R600, which only support GT, GE, NE, and EQ
condition codes as it removes the need to handle unsupported condition
codes in target specific code.

There are no tests with this commit, but R600 has been updated to take
advantage of this new feature, so its existing selectcc tests are now
testing the swapped operands path.
SelectionDAG: Improve legalization of SELECT_CC with illegal condition codes

SelectionDAG will now attempt to inverse an illegal conditon in order to
find a legal one and if that doesn't work, it will attempt to swap the
operands using the inverted condition.

There are no new test cases for this, but a nubmer of the existing R600
tests hit this path.
R600: Fix handling of NAN in comparison instructions

We were completely ignoring the unorder/ordered attributes of condition
codes and also incorrectly lowering seto and setuo.

Reviewed-by: Vincent Lejeune<vljn at ovi.com>