Revert "[IndVars] Smart hard uses detection"

This reverts commit 2f425e9c7946b9d74e64ebbfa33c1caa36914402.

It seems that the check that we still should do the transform if we
know the result is constant is missing in this code. So the logic that
has been deleted by this change is still sometimes accidentally useful.
I revert the change to see what can be done about it. The motivating
case is the following:

@Y = global [400 x i16] zeroinitializer, align 1

define i16 @foo() {
entry:
  br label %for.body

for.body:                                         ; preds = %entry, %for.body
  %i = phi i16 [ 0, %entry ], [ %inc, %for.body ]

  %arrayidx = getelementptr inbounds [400 x i16], [400 x i16]* @Y, i16 0, i16 %i
  store i16 0, i16* %arrayidx, align 1
  %inc = add nuw nsw i16 %i, 1
  %cmp = icmp ult i16 %inc, 400
  br i1 %cmp, label %for.body, label %for.end

for.end:                                          ; preds = %for.body
  %inc.lcssa = phi i16 [ %inc, %for.body ]
  ret i16 %inc.lcssa
}

We should be able to figure out that the result is constant, but the patch
breaks it.

Differential Revision: https://reviews.llvm.org/D51584

llvm-svn: 346198