target/ppc: Fix QEMU crash with stxsdx

I've been hitting several QEMU crashes while running a fedora29 ppc64le
guest under TCG. Each time, this would occur several minutes after the
guest reached login:

Fedora 29 (Twenty Nine)
Kernel 4.20.6-200.fc29.ppc64le on an ppc64le (hvc0)

Web console: https://localhost:9090/

localhost login:
tcg/tcg.c:3211: tcg fatal error

This happens because a bug crept up in the gen_stxsdx() helper when it
was converted to use VSR register accessors by commit 8b3b2d75c7
"target/ppc: introduce get_cpu_vsr{l,h}() and set_cpu_vsr{l,h}() helpers
for VSR register access".

The code creates a temporary, passes it directly to gen_qemu_st64_i64()
and then to set_cpu_vrsh()... which looks like this was mistakenly
coded as a load instead of a store.

Reverse the logic: read the VSR to the temporary first and then store
it to memory.

Fixes: 8b3b2d75c7
Signed-off-by: Greg Kurz <groug@kaod.org>
Message-Id: <155371035249.2038502.12364252604337688538.stgit@bahia.lan>
Reviewed-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

target/ppc/translate/vsx-impl.inc.c

@@ -356,8 +356,8 @@ static void gen_##name(DisasContext *ctx)                     \
     gen_set_access_type(ctx, ACCESS_INT);                     \
     EA = tcg_temp_new();                                      \
     gen_addr_reg_index(ctx, EA);                              \
+    get_cpu_vsrh(t0, xS(ctx->opcode));                        \
     gen_qemu_##operation(ctx, t0, EA);                        \
-    set_cpu_vsrh(xS(ctx->opcode), t0);                        \
     tcg_temp_free(EA);                                        \
     tcg_temp_free_i64(t0);                                    \
 }