gh-146381: Fold frozendict(key=const, ...) calls to LOAD_CONST

[corona10]

This PR folds frozendict calls into LOAD_CONST at compile time.

This is a prerequisite for JIT constant folding of frozendict lookups. Without this, frozendict compiles to CALL_KW, which the JIT cannot recognize as a constant. With this change, it becomes LOAD_CONST frozendict(...), enabling the JIT to promote it and fold subscript operations like d['x'] at optimization time.

Without codegen-level folding like this, supporting frozendict constants in the JIT would likely require introducing a new literal syntax (e.g., <1: 3, 5: 6>) at the language level, and it might need a PEP for this change.

cc @vstinner

[corona10]

## main
5           LOAD_GLOBAL              1 (frozendict + NULL)
LOAD_SMALL_INT           1
LOAD_SMALL_INT           2
LOAD_CONST               1 (('x', 'y'))
CALL_KW                  2
STORE_FAST               0 (d)


## gh-146381
5           LOAD_GLOBAL              0 (frozendict)
COPY                     1
LOAD_COMMON_CONSTANT     7 (frozendict)
IS_OP                    0 (is)
POP_JUMP_IF_FALSE        4 (to L1)
NOT_TAKEN
POP_TOP
LOAD_CONST               0 (frozendict({'x': 1, 'y': 2}))  ← compile-time constant
JUMP_FORWARD             8 (to L2)
L1:     PUSH_NULL
LOAD_SMALL_INT           1
LOAD_SMALL_INT           2
LOAD_CONST               1 (('x', 'y'))
CALL_KW                  2                                  ← fallback
L2:     STORE_FAST               0 (d)

[corona10]

Since @Fidget-Spinner suggeted to optimize from JIT pipeline directly, I covert this PR to the draft.