target/arm: Split out aarch32_cpsr_valid_mask

target/arm: Split out aarch32_cpsr_valid_mask

Split this helper out of msr_mask in translate.c.  At the same time,
transform the negative reductive logic to positive accumulative logic.
It will be usable along the exception paths.

While touching msr_mask, fix up formatting.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20200208125816.14954-6-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>