[SDAG] commute setcc operands to match a subtract

If we have:

R = sub X, Y
P = cmp Y, X

...then flipping the operands in the compare instruction can allow using a subtract that sets compare flags.

Motivated by diffs in D58875 - not sure if this changes anything there,
but this seems like a good thing independent of that.

There's a more involved version of this transform already in IR (in instcombine
although that seems misplaced to me) - see "swapMayExposeCSEOpportunities()".

Differential Revision: https://reviews.llvm.org/D63958

llvm-svn: 365711