Co-authored-by: Alan Wu <alanwu@ruby-lang.org>

* YJIT: Local variable register allocation

* locals are not stack temps

* Rename RegTemps to RegMappings

* Rename RegMapping to RegOpnd

* Rename local_size to num_locals

* s/stack value/operand/

* Rename spill_temps() to spill_regs()

* Clarify when num_locals becomes None

* Mention that InsnOut uses different registers

* Rename get_reg_mapping to get_reg_opnd

* Resurrect --yjit-temp-regs capability

* Use MAX_CTX_TEMPS and MAX_CTX_LOCALS

Previously, PosMarker callbacks ran even when the assembler failed to
assemble its contents due to insufficient space. This was problematic
because when Assembler::compile() failed, the callbacks were given
positions that have no valid code, contrary to general expectation.

For example, we use a PosMarker callback to record VM instruction
boundaries and patch in jumps to exits in case the guest program starts
tracing, however, previously, we could record a location near the end of
the code block, where there is no space to patch in jumps. I suspect
this is the cause of the recent occurrences of rare random failures on
GitHub Actions with the invariants.rs:529 "can rewrite existing code"
message. `--yjit-perf` also uses PosMarker and had a similar issue.

Buffer the list of callbacks to fire, and only fire them when all code
in the assembler are written out successfully. It's more intuitive this
way.

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).

* YJIT: Skip Insn::Comment and format!

if disasm is disabled

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

* YJIT: Get rid of asm.comment

---------

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

* YJIT: Introduce Target::SideExit

* YJIT: Obviate Insn::SideExitContext

* YJIT: Avoid cloning a Context for each insn

Co-authored-by: Alan Wu <alansi.xingwu@shopify.com>
Co-authored-by: Maxime Chevalier-Boisvert <maxime.chevalierboisvert@shopify.com>