[PowerPC] Handle cmp op promotion for SELECT[_CC] nodes in PPCTL::DAGCombineExtBoolTrunc

PPCTargetLowering::DAGCombineExtBoolTrunc contains logic to remove unwanted
truncations and extensions when dealing with nodes of the form:
  zext(binary-ops(binary-ops(trunc(x), trunc(y)), ...)

There was a FIXME in the implementation (now removed) regarding the fact that
the function would abort the transformations if any of the non-output operands
of a SELECT or SELECT_CC node would need to be promoted (because they were
also output operands, for example). As a result, we continued to generate
unnecessary zero-extends for code such as this:

  unsigned foo(unsigned a, unsigned b) {
    return  (a <= b) ? a : b;
  }

which would produce:

  cmplw 0, 3, 4
  isel 3, 4, 3, 1
  rldicl 3, 3, 0, 32
  blr

and now we produce:

  cmplw 0, 3, 4
  isel 3, 4, 3, 1
  blr

which is better in the obvious way.