decodetree: Propagate return value from translate subroutines

decodetree: Propagate return value from translate subroutines

Allow the translate subroutines to return false for invalid insns.

At present we can of course invoke an invalid insn exception from within
the translate subroutine, but in the short term this consolidates code.
In the long term it would allow the decodetree language to support
overlapping patterns for ISA extensions.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20180227232618.2908-1-richard.henderson@linaro.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>