regex: special case literal extraction

In a prior commit, we fixed a performance problem with the -w flag by
doing a little extra work to extract literals. It turns out that using
literals in this case when the -w flag is NOT used results in a
performance regression. The reasoning is that we end up using a "fast"
regex as a prefilter when the regex engine itself uses its own
equivalent prefilter, so ripgrep ends up redoing a fair amount of work.

Instead, we only do this extra work when we know the -w flag is enabled.

crates/regex/src/literal.rs

@@ -141,6 +141,9 @@ impl LiteralSets {
             // (Not in theory---it could be better. But the current
             // implementation isn't good enough.) ... So we make up for it
             // here.
+            if !word {
+                return None;
+            }
             let p_min_len = self.prefixes.min_len();
             let s_min_len = self.suffixes.min_len();
             let lits = match (p_min_len, s_min_len) {