Fix more multibyte issues in ltree.

Fix more multibyte issues in ltree.

Commit 84d5efa7e3 missed some multibyte issues caused by short-circuit
logic in the callers. The callers assumed that if the predicate string
is longer than the label string, then it couldn't possibly be a match,
but it can be when using case-insensitive matching (LVAR_INCASE) if
casefolding changes the byte length.

Fix by refactoring to get rid of the short-circuit logic as well as
the function pointer, and consolidate the logic in a replacement
function ltree_label_match().

Discussion: https://postgr.es/m/02c6ef6cf56a5013ede61ad03c7a26affd27d449.camel@j-davis.com
Backpatch-through: 14