strings: adding micro-optimization for TrimSpace

replace for string's end trimming TrimFunc -> TrimRightFunc

strings.TrimSpace string's end trimming should use more specific TrimRightFunc instead of common TrimFunc (because start has already trimmed before)

Change-Id: I827f1a25c141e61edfe1f8b11f6e8cd685f8b384
GitHub-Last-Rev: 040607a
GitHub-Pull-Request: golang#46862
Reviewed-on: https://go-review.googlesource.com/c/go/+/329731
Auto-Submit: Russ Cox <rsc@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

src/strings/strings.go

@@ -962,7 +962,8 @@ func TrimSpace(s string) string {
 	for ; stop > start; stop-- {
 		c := s[stop-1]
 		if c >= utf8.RuneSelf {
-			return TrimFunc(s[start:stop], unicode.IsSpace)
+			// start has been already trimmed above, should trim end only
+			return TrimRightFunc(s[start:stop], unicode.IsSpace)
 		}
 		if asciiSpace[c] == 0 {
 			break