cases: add bit for exception index

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Updates golang/go#27945

Change-Id: I5add868e60ba632de61b1ff3cb520453759b95c4
Reviewed-on: https://go-review.googlesource.com/c/text/+/169639
Run-TryBot: Marcel van Lohuizen <mpvl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

cases/gen.go

@@ -335,7 +335,7 @@ func makeException(ri *runeInfo) {
 	ri.entry |= exceptionBit
 
 	if len(exceptionData) >= 1<<numExceptionBits {
-		log.Fatalf("%U:exceptionData too large %x > %d bits", ri.Rune, len(exceptionData), numExceptionBits)
+		log.Fatalf("%U:exceptionData too large %#x > %d bits", ri.Rune, len(exceptionData), numExceptionBits)
 	}
 
 	// Set the offset in the exceptionData array.

cases/gen_trieval.go

@@ -19,8 +19,7 @@ package main
 // The per-rune values have the following format:
 //
 //   if (exception) {
-//     15..5  unsigned exception index
-//         4  unused
+//     15..4  unsigned exception index
 //   } else {
 //     15..8  XOR pattern or index to XOR pattern for case mapping
 //            Only 13..8 are used for XOR patterns.
@@ -52,8 +51,8 @@ const (
 	isMidBit       = 1 << 6
 
 	exceptionBit     = 1 << 3
-	exceptionShift   = 5
-	numExceptionBits = 11
+	exceptionShift   = 4
+	numExceptionBits = 12
 
 	xorIndexBit = 1 << 6
 	xorShift    = 8