topdown: Avoid unnecessary work in biunify.

Most notable changes:

benchmark                                 old ns/op      new ns/op      delta
BenchmarkVirtualCache-8                   330            317            -3.94%
BenchmarkLargeJSON-8                      55162595       53561595       -2.90%
BenchmarkConcurrency1-8                   219100992      219464996      +0.17%
BenchmarkConcurrency2-8                   121519274      119700122      -1.50%
BenchmarkConcurrency4-8                   88765327       79863590       -10.03%
BenchmarkConcurrency8-8                   93624724       84036312       -10.24%
BenchmarkConcurrency4Readers1Writer-8     90931759       82038964       -9.78%
BenchmarkConcurrency8Writers-8            237043373      233894508      -1.33%
BenchmarkVirtualDocs1x1-8                 8307           8057           -3.01%
BenchmarkVirtualDocs10x1-8                8211           8133           -0.95%
BenchmarkVirtualDocs100x1-8               8556           8497           -0.69%
BenchmarkVirtualDocs1000x1-8              9416           9329           -0.92%
BenchmarkVirtualDocs10x10-8               31243          30562          -2.18%
BenchmarkVirtualDocs100x10-8              32564          31880          -2.10%
BenchmarkVirtualDocs1000x10-8             36140          34982          -3.20%
BenchmarkPartialEvalCompile/1-8           3532920        3545738        +0.36%
BenchmarkPartialEvalCompile/10-8          4835445        4837658        +0.05%
BenchmarkPartialEvalCompile/100-8         30517705       30033534       -1.59%
BenchmarkPartialEvalCompile/1000-8        1670470326     1627247427     -2.59%

Signed-off-by: Teemu Koponen <koponen@styra.com>