When doing completing prompt with REQUIRE-MATCH? set true, delay

When doing completing prompt with REQUIRE-MATCH? set true, delay
running completion procedure on final value.  First try VERIFY-MATCH?
procedure, and if it succeeds, don't do any completion.  This
shouldn't change the behavior since VERIFY-MATCH? must be run anyway,
and if the result is already a match, completion does nothing.

This is helpful when the completion procedure is expensive but
VERIFY-MATCH? is relatively cheap.