target/arm: Rearrange {sve,fp}_check_access assert

target/arm: Rearrange {sve,fp}_check_access assert

We want to ensure that access is checked by the time we ask
for a specific fp/vector register.  We want to ensure that
we do not emit two lots of code to raise an exception.

But sometimes it's difficult to cleanly organize the code
such that we never pass through sve_check_access exactly once.
Allow multiple calls so long as the result is true, that is,
no exception to be raised.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20200815013145.539409-5-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>