Fix regexp set matches for literal matchers

I was surprised to find out that posting lookups for `foo=~"(bar|bar)"`
are faster than `foo=~"bar"`. It turns out we introduced a performance
regression in https://github.com/grafana/mimir-prometheus/pull/463.

When we added the `optimizeAlternatingLiterals` function, we subtly
broke one edge case. A regexp matcher which matches a single literal,
like `foo=~"bar"` used to return `bar` from `SetMatches()`, but
currently does not. The implication is that the tsdb will first do a
LabelValues call to get all values for `foo`, then match them against
the regexp `bar`. This PR restores the previous behavior which is able
to directly lookup postings for `foo="bar"` instead.

model/labels/regexp.go

@@ -366,7 +366,7 @@ func optimizeAlternatingLiterals(s string) (StringMatcher, []string) {
 	// If there are no alternates, check if the string is a literal
 	if estimatedAlternates == 1 {
 		if regexp.QuoteMeta(s) == s {
-			return &equalStringMatcher{s: s, caseSensitive: true}, nil
+			return &equalStringMatcher{s: s, caseSensitive: true}, []string{s}
 		}
 		return nil, nil
 	}

model/labels/regexp_test.go

@@ -342,6 +342,14 @@ func TestFindSetMatches(t *testing.T) {
 			matches, actualCaseSensitive := findSetMatches(parsed)
 			require.Equal(t, c.expMatches, matches)
 			require.Equal(t, c.expCaseSensitive, actualCaseSensitive)
+
+			if c.expCaseSensitive {
+				// When the regexp is case sensitive, we want to ensure that the
+				// set matches are maintained in the final matcher.
+				r, err := newFastRegexMatcherWithoutCache(c.pattern)
+				require.NoError(t, err)
+				require.Equal(t, c.expMatches, r.SetMatches())
+			}
 		})
 	}
 }