target/arm: Split out subroutines of handle_shri_with_rndacc

target/arm: Split out subroutines of handle_shri_with_rndacc

There isn't a lot of commonality along the different paths of
handle_shri_with_rndacc.  Split them out to separate functions,
which will be usable during the decodetree conversion.

Simplify 64-bit rounding operations to not require double-word arithmetic.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20240912024114.1097832-22-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>