Revert "[SimpleLoopUnswitch] unswitch selects"

This reverts commit 21f226fc. Crashes on
this test case:

define void @test2() nounwind {
entry:
  br label %bb.nph

bb.nph:                                           ; preds = %entry
  %and.i13521 = and <4 x i1> undef, undef
  br label %for.body

for.body:                                         ; preds = %for.body, %bb.nph
  %or.i = select <4 x i1> %and.i13521, <4 x i32> undef, <4 x i32> undef
  br i1 false, label %for.body, label %for.end

for.end:                                          ; preds = %for.body, %entry
  ret void
}