Call a hybrid function cast from a plain function directly

[ASDAlexander77]

Problem

Casting a plain function value (e.g. an address from GetProcAddress) to a function type produces a hybrid function whose this is a literal null. Every call through it was still lowered to a runtime this test with two call paths and a phi, at every optimization level. The dead path even called the plain function with an extra ptr null argument.

br i1 false, label %2, label %4
2: %3 = call double @add1(ptr null, double 4.1e+01)   ; dead, wrong arity
4: %5 = call double @add1(double 4.1e+01)
6: %7 = phi double [ %5, %4 ], [ %3, %2 ]

Fix

SimplifyIndirectCallWithKnownCallee now handles a callee defined by ts.Cast from FunctionType to HybridFunctionType: it calls the cast's input directly and erases the cast if it is unused.

• The canonicalizer runs regardless of --opt, so --opt_level=0 / --di builds benefit.
• It runs before LowerToAffine turns calls inside try into ts.Invoke, so one pattern covers both call and invoke; no LLVM lowering changes.
• Function-typed variables keep the runtime check, since they may hold a bound method.

Result at --opt_level=0: %2 = call double @add1(double 4.1e+01).

Design notes and rejected alternatives: docs/hybrid-func-null-this-call-optimization.md.

Tests

• New 00funcs_hybrid_null_this.ts: direct call, call inside try, and a bound method through a function-typed variable. Registered as compile, jit and rc/none corpus; all 6 pass.
• Full Windows debug suite: 2698/2715. The 17 failures are pre-existing: 14 were already failing before this branch, and the 3 gc-shared-auto / gc-defaultlib-collector failures (--emit=dll exits 0x80000003) reproduce with this change reverted.

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