target/s390x: move tcg_gen_insn_start to s390x_tr_insn_start

We use INDEX_op_insn_start to make the start of instruction boundaries.
If we don't do it in the .insn_start hook things get confused especially
now plugins want to use that marking to identify the start of instructions
and will bomb out if it sees instrumented ops before the first instruction
boundary.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20211011185332.166763-1-richard.henderson@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>