target/arm: Fix bugs in MVE VRMLALDAVH, VRMLSLDAVH

target/arm: Fix bugs in MVE VRMLALDAVH, VRMLSLDAVH

The initial implementation of the MVE VRMLALDAVH and VRMLSLDAVH
insns had some bugs:
 * the 32x32 multiply of elements was being done as 32x32->32,
   not 32x32->64
 * we were incorrectly maintaining the accumulator in its full
   72-bit form across all 4 beats of the insn; in the pseudocode
   it is squashed back into the 64 bits of the RdaHi:RdaLo
   registers after each beat

In particular, fixing the second of these allows us to recast
the implementation to avoid 128-bit arithmetic entirely.

Since the element size here is always 4, we can also drop the
parameterization of ESIZE to make the code a little more readable.

Suggested-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20210628135835.6690-3-peter.maydell@linaro.org