target/arm: Vectorize USHL and SSHL

target/arm: Vectorize USHL and SSHL

These instructions shift left or right depending on the sign
of the input, and 7 bits are significant to the shift.  This
requires several masks and selects in addition to the actual
shifts to form the complete answer.

That said, the operation is still a small improvement even for
two 64-bit elements -- 13 vector operations instead of 2 * 7
integer operations.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20200216214232.4230-2-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>