(for callee saved regs, we insert push/pop before reg alloc. and
after allocation, remove unnecessary ones)

we never go back to previous passes, since it is not necessary

convention(stack args), and need to optimize spill1 code

Client may pass a name with AT or PERCENTAGE, but internally MuName
does not contain that.

Ensure that "structs" have names, so that structs can be referred
symbolically. In the future, functions and basic blocks also need to be
ensured to have names. Such names are generated from their IDs if not
provided by the client.

The IR builder can create named or unnamed MuEntityHeader based on
whether it has client-provided name or not.

Can build constants now (currently only Constant::Int).

Also resolves dependency

NOTE: Only fixed hand-written ASMs. Emitted ".s" files still have problems.

Implemented dependency resolution.

now we have 4 passes: fast_inst_sel, fast_reg_alloc, followed
by slow_inst_sel and slow_reg_alloc. This might make the
compilation pipeline more linear

Creaation and destruction of MuVM, MuCtx and MuIRBuilder. Now the
`mvm->new_context`, `ctx->new_ir_builder`, `ctx->close_context` and
`b->abort` works. MuVM can also be created via a top-level "C"-style
exported function `mu_fastimpl_new()` so that in theory, it can be used
in C programs.