revpair: do not optimize tree to check for odd-range spec
At
cc3a30ff9490, we had to optimize a parsed tree to resolve x^:y ambiguity.
Since we've moved the resolution of x^:y to parse(), we no longer have to call
optimize(). Therefore, (x:y) can be taken as a single expression, not an odd
range expression x:y.
#require docutils gettext
$ $TESTDIR/check-gendoc it
checking for parse errors