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.

llvm-svn: 191600