tcg/ppc: Use TCG_REG_TMP2 for scratch index in prepare_host_addr

tcg/ppc: Use TCG_REG_TMP2 for scratch index in prepare_host_addr

In tcg_out_qemu_ldst_i128, we need a non-zero index register,
which we then use as a base register in several address modes.
Since we always have TCG_REG_TMP2 available, use that.

Cc: qemu-stable@nongnu.org
Fixes: 526cd4ec01f ("tcg/ppc: Support 128-bit load/store")
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2597
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Tested-By: Michael Tokarev <mjt@tls.msk.ru>