tcg: correct 32-bit tcg_gen_ld8s_i64 sign-extension

The version of tcg_gen_ld8s_i64 for 32-bit systems does a load into
the low part of the return value - then attempts a sign extension into
the high part, but wrongly sets the high part to a sign extension of
itself rather than of the low part.  This results in TCG internal
errors from the use of the uninitialized high part (in some GCC tests
of AArch64 NEON shift intrinsics, in particular).  This patch corrects
the sign-extension logic, making it match other functions such as
tcg_gen_ld16s_i64.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Joseph Myers <joseph@codesourcery.com>
Message-Id: <alpine.DEB.2.20.1610272333560.22353@digraph.polyomino.org.uk>
Signed-off-by: Richard Henderson <rth@twiddle.net>
