[PowerPC] Remove dead code from PPCDAGToDAGISel::SelectSETCC

The subroutine getCRIdxForSetCC has a parameter "Other" and comment:

  If this returns with Other != -1, then the returned comparison
  is an or of two simpler comparisons.

However for at least the last five years this routine has never
returned a value of Other != -1; these cases are now handled
differently to begin with.

This patch removes the parameter and the code in SelectSETCC that
attempted to handle the Other != -1 case.