tcg/aarch64: Do not convert TCGArg to temps that are not temps

Fixes INDEX_op_rotli_vec for aarch64 host, where the 3rd
argument is an integer, not a temporary, which now tickles
an assert added in e89b28a6.

Previously, the value computed into v2 would be garbage for
rotli_vec, but as the value was unused it caused no harm.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>