Fix function pointer assignment

[ChAoSUnItY]

This patch fixes function pointer assignment on local variable, direct struct member, and pointer-to-struct member, and also fixes in-register arguments not cleared after invocation of function pointer.

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Summary by cubic

Fixes incorrect function-pointer assignment and cleans up register liveness after indirect calls to prevent wrong-address reads/writes and spill corruption. Previously locals and struct members could target the wrong address and call-argument registers stayed live after function-pointer calls; now locals invoke the pointer directly, member stores target the base address, and registers are cleared post-call.

Bug Fixes

• Skip the load when calling a non-reference local function pointer.
• Store function pointers to struct or pointer-to-struct members by taking the base address before the write.
• Target OP_address_of at aggregate backing storage, not the spill slot, and allocate backing storage for parameter and nested aggregates with function-pointer fields.
• Clear live variable mappings for all registers after indirect calls.
• Add tests for locals, struct and pointer-to-struct members, parameter and nested aggregates, and stale argument-register cleanup.

^{Written for commit 0e76361. Summary will update on new commits.}

[DrXiao]

How about adding some comments?

[DrXiao]

Ditto.