Minor cleanup in ComputeScanRangeAssignment()

1) Rather than using initial_bytes as a signal for whether there exists
a co-located backend, be more direct about this.

2) Don't use a mix of both '&&' and an if-stmt to compute a boolean
value.  It's easier to read if we just use boolean operators.

Change-Id: I9d780dd6ad3833058e420b0654543da6e669f364
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/5476
Reviewed-by: Alex Behm <alex.behm@cloudera.com>
Tested-by: jenkins