gh-143820: Fix assertion failure that occurred when inserting ENTER_EXECUTOR after BINARY_OP_INPLACE_ADD_UNICODE

[cocolato]

This PR calculate the target STORE_FAST local variable index during the Specialization phase and store it in the instruction's own inline cache.

So that:

1. The instruction becomes entirely self-contained at runtime. It no longer cares if the JIT subsequently overwrites the next instruction with ENTER_EXECUTOR.
2. It correctly handles cases where the STORE_FAST has an index > 255 (which uses EXTENDED_ARG prefixes).

• Issue: JIT: Assertion next_instr->op.code == STORE_FAST failed in _PyEval_EvalFrameDefault #143820

[Fidget-Spinner]

Wow, this is incredibly smart! I like this solution! Thanks!