target/i386: Destructive vector helpers for AVX

target/i386: Destructive vector helpers for AVX

These helpers need to take special care to avoid overwriting source values
before the wole result has been calculated.  Currently they use a dummy
Reg typed variable to store the result then assign the whole register.
This will cause 128 bit operations to corrupt the upper half of the register,
so replace it with explicit temporaries and element assignments.

Signed-off-by: Paul Brook <paul@nowt.org>
Message-Id: <20220424220204.2493824-14-paul@nowt.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>