target/i386: Destructive FP helpers for AVX

target/i386: Destructive FP helpers for AVX

Perpare the horizontal atithmetic vector helpers for AVX
These currently 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: <20220424220204.2493824-18-paul@nowt.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>