cmd/compile: test for lower-case letters first in isAlpha

Lower-case letters are more common in identifiers.

Change-Id: I49c39e3ac810eea57d15c1433608daec212c9792
Reviewed-on: https://go-review.googlesource.com/19760Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/gc/lex.go

@@ -832,7 +832,7 @@ func isSpace(c int) bool {
 }
 
 func isAlpha(c int) bool {
-	return 'A' <= c && c <= 'Z' || 'a' <= c && c <= 'z'
+	return 'a' <= c && c <= 'z' || 'A' <= c && c <= 'Z'
 }
 
 func isDigit(c int) bool {