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 <[email protected]>
Signed-off-by: Richard Henderson <[email protected]>
Message-id: 20200216214232[email protected]
Signed-off-by: Peter Maydell <[email protected]>