fixing dce pass to remove duplicate shape queries.

[jjsjann123]

What does this PR do?

dce pass doesn't remove duplicate prims.shape node.

  # /volume/ttt.py:9:       shape_0 = x.shape
  (i0, i1, i2, i3) = prims.shape(x)
  # /volume/ttt.py:10:      shape_1 = x.shape
  (i0, i1, i2, i3) = prims.shape(x)

becomes

  # /volume/ttt.py:9:       shape_0 = x.shape
  (i0, i1, i2, i3) = prims.shape(x)
  # /volume/ttt.py:10:      shape_1 = x.shape
  (_, _, _, _) = prims.shape(x)

This PR patches it to instead emit

  # /volume/ttt.py:9:       shape_0 = x.shape
  (i0, i1, i2, i3) = prims.shape(x)

Helps #1579 .

Note that I don't think this fixes the whole issue, but I think it's a more consistent behavior, since dce is expected to remove un-used operations.

[jjsjann123]

tagging @t-vi for review. also cc'ing @tfogal