fix(cgen): fix address-of and as_cast pointer codegen

[guweigang]

Summary

This PR fixes a C code generation issue where combining the address-of (&) operator and as_cast expressions (specifically during smartcasts) results in incorrect pointer casting and dereferencing syntax in the generated C output.

Details of Changes

In vlib/v/gen/c/cgen.v:

1. Gen.expr:
   • Enhanced the address-of (&) operator codegen path. When the right-hand side is an AsCast expression, we check if it evaluates to a pointer casting using the new as_cast_will_use_ptr() helper. If so, we avoid writing incorrect prefix operators (like double address-of or invalid dereference syntax).
2. Gen.write_as_cast_call_start:
   • Added a check for g.is_amp (address-of context) to ensure the compiler generates (Type*) pointer casts rather than standard value casts when smartcasting is performed in an address-of context.
3. Gen.as_cast_will_use_ptr:
   • Introduced this helper method to accurately determine if an AsCast node will result in a pointer-based representation under the hood (e.g. casting interfaces or sum types with differing base types).

Checklist

• I have formatted my V files with v fmt -w.
• All existing tests pass.

Tests Run

Rebuilt the V compiler and verified that all compiler and cgen tests pass:

./v -g -keepc -o ./vnew cmd/v
./vnew -silent test vlib/v/gen/c/

All tests passed successfully on local machine.

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