Eliminate unnecessary rerun of test_width in case statement elaboration.

Also fix a typo in a comment.
Added pruning of case statement expressions.

When unsized literals are used in case item expressions, it is likely
that the calculated expression width will be larger than necessary to
unambiguously select the correct case item (particularly when using
"strict" expression elaboration). This patch adds an optimisation
step that prunes the expressions to the minimum necessary width.