tcg/optimize: Fix expansion/simplification of deposit

When we computed z_mask/o_mask for the deposit, we may accidentally
prove that the result is a constant.

If we let the rest of the code expand as-is, when can we get to the
end we'll see the constant, note that it's the same as the current
value, remove what we think is "the opcode" that computes this same
value.  But in this case we've been expanding deposit to multiple
opcodes and the removal of only the last opcode breaks the expansion.

Fix this by noting the constant result early, before expanding,
allowing the entire operation to be removed.

Cc: qemu-stable@nongnu.org
Fixes: 5f747705a4 ("tcg/optimize: Lower unsupported deposit during optimize")
Resolves: https://gitlab.com/qemu-project/qemu/-/work_items/4449
Reviewed-by: Jim MacArthur <jim.macarthur@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
(cherry picked from commit 0bc331d1c7d5cedc1dbe0bf21dc5f6be13c58fa4)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>