BUG: Fix fusion optimization bug that incorrectly changes operation order

[icexelloss]

What change is proposed

Example code

t = ibis.table(ibis.schema([('col', 'float32')]), 't')
t = t[['col']]
t = t[~t['col'].isnan()]
t = t.mutate(col=t['col'].cast('int32'))

Before:

ref_0
UnboundTable[table]
  name: t
  schema:
    col : float32

Selection[table]
  table:
    Table: ref_0
  selections:
    col = Cast[int32*]
      col = Column[float32*] 'col' from table
        ref_0
      to:
        int32
  predicates:
    Not[boolean*]
      IsNan[boolean*]
        col = Column[float32*] 'col' from table
          ref_0

After:

ref_0
UnboundTable[table]
  name: t
  schema:
    col : float32

ref_1
Selection[table]
  table:
    Table: ref_0
  selections:
    col = Column[float32*] 'col' from table
      ref_0
  predicates:
    Not[boolean*]
      IsNan[boolean*]
        col = Column[float32*] 'col' from table
          ref_0

Selection[table]
  table:
    Table: ref_1
  selections:
    col = Cast[int32*]
      col = Column[float32*] 'col' from table
        ref_1
      to:
        int32

Testing

I added a few tests in test_analysis.py to assert the fusion behavior. Also created a test in backends test to make sure the results are expected.

[jreback]

can you also add a release note

[jreback]

thanks @icexelloss

[icexelloss]

Thanks @jreback !