Remove mutability tracking from MIR semantics

[Stevengre]

Motivation

PR #948 removed the mutabilityOf(...) ==K mutabilityMut guard from #setLocalValue, because MIR's LocalDecl::mutability is a source-level annotation (let x vs let mut x), not an assignment constraint. The Rust compiler's borrow checker validates assignment legality before emitting MIR, so all assignments in well-formed MIR are valid regardless of the mutability field.

After #948, #forceSetLocal is functionally identical to #setLocalValue — both write without checking mutability.

Proposal

Remove mutability tracking from the MIR semantics definition entirely:

• Remove mutabilityMut / mutabilityNot from typedValue and related data structures
• Unify #forceSetLocal into #setLocalValue (or remove #forceSetLocal)
• Clean up any remaining mutability checks or propagation throughout the codebase

References

• PR fix(rt): remove mutability guard on local variable assignment #948: fix(rt): remove mutability guard on local variable assignment
• LocalDecl::mutability docs: "Whether this is a mutable binding (i.e., let x or let mut x). Temporaries and the return place are always mutable."

[Stevengre]

BTW, it may also a bug that we should report to rust community. If they want to make sure that mutability has meaning at stable mir level, they sould change mutability from not to mut when they want to reuse a variable.