target/arm: Create gen_gvec_[us]sra

target/arm: Create gen_gvec_[us]sra

The functions eliminate duplication of the special cases for
this operation.  They match up with the GVecGen2iFn typedef.

Add out-of-line helpers.  We got away with only having inline
expanders because the neon vector size is only 16 bytes, and
we know that the inline expansion will always succeed.
When we reuse this for SVE, tcg-gvec-op may decide to use an
out-of-line helper due to longer vector lengths.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20200513163245.17915-2-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>