target/i386: use TSTEQ/TSTNE to test low bits

When testing the sign bit or equality to zero of a partial register, it
is useful to use a single TSTEQ or TSTNE operation.  It can also be used
to test the parity flag, using bit 0 of the population count.

Do not do this for target_ulong-sized values however; the optimizer would
produce a comparison against zero anyway, and it avoids shifts by 64
which are undefined behavior.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

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