We've long had a size restriction on the code memory region such that a
u32 could refer to everything. This commit capitalizes on this
restriction by shrinking the size of `CodePtr` to be 4 bytes from 8.

To derive a full raw pointer from a `CodePtr`, one needs a base pointer.
Both `CodeBlock` and `VirtualMemory` can be used for this purpose. The
base pointer is readily available everywhere, except for in the case of
the `jit_return` "branch". Generalize lea_label() to lea_jump_target()
in the IR to delay deriving the `jit_return` address until `compile()`,
when the base pointer is available.

On railsbench, this yields roughly a 1% reduction to `yjit_alloc_size`
(58,397,765 to 57,742,248).

So that we get a reminder to check CodeBlock::has_dropped_bytes().
Internally, asm.compile() already checks it, and this patch just
propagates it out to the caller with a `#[must_use]`.

Code GC logic moved out one level in entry_stub_hit(), so the body
can freely use `?`

> note: `#[deny(clippy::redundant_locals)]` on by default

On Rust 1.73.0.

Previously, at the end of `leave` we did
`*caller_cfp->sp = return_value`, like the interpreter.
With future changes that leaves the SP field uninitialized for C frames,
this will become problematic. For cases like returning from
`rb_funcall()`, the return value was written above the stack and
never read anyway (callers use the copy in the return register).

Leave the return value in a register at the end of `leave` and have the
code at `cfp->jit_return` decide what to do with it. This avoids the
unnecessary memory write mentioned above. For JIT-to-JIT returns, it goes
through `asm.stack_push()` and benefits from register allocation for
stack temporaries.

Mostly flat on benchmarks, with maybe some marginal speed improvements.

Co-authored-by: Takashi Kokubun <takashikkbn@gmail.com>

* YJIT: Chain-guard opt_mult overflow

* YJIT: Support regenerating Jo after Mul

* YJIT: Avoid creating a vector in get_temp_regs()

Co-authored-by: Alan Wu <alansi.xingwu@shopify.com>

* Remove unused import

---------

Co-authored-by: Alan Wu <alansi.xingwu@shopify.com>
Co-authored-by: Alan Wu <XrXr@users.noreply.github.com>