[XLA:GPU] Create backwards-input convs when the filter is a constant.

Previously, we'd only create a backwards conv if the filters was 1x1 or
a kReverse.  But if the user ever wants to do a backwards-input conv
with the filter being a constant, there's no way for them to arrange for
the filter to be a kReverse: By the time we see it in the convolution
rewriter, the reverse will have been constant-folded away.

PiperOrigin-RevId: 210632241