decodetree: Fix codegen for non-overlapping group inside overlapping group

For nested groups like:

{
[
pattern 1
pattern 2
]
pattern 3
}

the intended behaviour is that patterns 1 and 2 must not
overlap with each other; if the insn matches neither then
we fall through to pattern 3 as the next thing in the
outer overlapping group.

Currently we generate incorrect code for this situation,
because in the code path for a failed match inside the
inner non-overlapping group we generate a "return" statement,
which causes decode to stop entirely rather than continuing
to the next thing in the outer group.

Generate a "break" instead, so that decode flow behaves
as required for this nested group case.

Backports 514101c0b931f0a11a40d29d26af1cc40482f951

qemu/scripts/decodetree.py

@@ -548,7 +548,7 @@ class Tree:
             output(ind, '    /* ',
                    str_match_bits(innerbits, innermask), ' */\n')
             s.output_code(i + 4, extracted, innerbits, innermask)
-            output(ind, '    return false;\n')
+            output(ind, '    break;\n')
         output(ind, '}\n')
 # end Tree