gh-145742: Manually emit _LOAD_FAST_BORROW to reduce stencil bloat

[corona10]

• Manually emit _LOAD_FAST_BORROW at JIT compile time, encoding the operand offset directly into the instruction instead of loading it from the GOT at runtime.
• This shrinks the generic case (oparg ≥ 8) from 28 bytes to 8 bytes and eliminates 27 stencil functions.
• I've compared machine code through godbolt:
  • x86-64 / aarch64: https://godbolt.org/z/3M9zKeosj
  • i686: https://godbolt.org/z/cdjdzev5Y

• Issue: Some (mostly) easy ways to reduce the size of JIT generated code #145742

[corona10]

For i686: https://godbolt.org/z/cdjdzev5Y

[diegorusso]

Some initial feedback on this:

• we should not to pollute jit.c with uops implementation. They should live in separate compile units and have the same signature of the other ones (e.g.: void emit__UOP_NAME(unsigned char *code, unsigned char *data, _PyExecutorObject *executor, const _PyUOpInstruction *instruction, jit_state *state)
• ifdefs can select the right architecture of the custom implementation
• in bytecodes.c we should have a way to tell the JIT machinery not to generated any code for a specific uops but the uops implemetation should be accounted in the table in the jit-stencils-*.h (static const StencilGroup stencil_groups[MAX_UOP_REGS_ID + 1])
• The linker later on will pick up our own version of the uops implementation.

[corona10]

Thanks, @diegorusso. I’ll keep working on this based on your feedback.