Currently the kWasmInstanceOffset is computed according to the reg
a0(kWasmInstanceRegister)'s position in the frame. And according
to Builtins::Generate_WasmCompileLazy, it's the 7th gp_regs that
are pushed on to stack, so the index should be 6 other than 7.

Since the kWasmInstanceRegister will be pushed on to stack after
all parameter registers, so we can use it's index, which does not
reply on which reg kWasmInstanceRegister is, and what order the
parameter registers are pushed on to stack.

So the new index is equal to the number of all parameter registers.

Change-Id: I7a77fb052a5d68ee28dab10409462260ad491578
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2425329
Commit-Queue: Zhao Jiazhong <zhaojiazhong-hf@loongson.cn>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70087}