Reapply "[Aarch64] [ISel] Don't save vaargs registers if vaargs are unused" (#131459) This reapplies original commit e122483762b44c7f4386165099ff2a404705d7d4 Second try, but with win64 removed from the scope, since it was somehow broken by original commit. ### Original PR description If vaargs are not used there is no need to save them. LLVM already implements such optimization for x86, as well as gcc [1]. Some ABI tests are kept almost as-is, except for stack offsets, by just adding llvm.va_start. Only laapcs_vararg_frame.ll test was rewritten to match new behavior. [1] https://godbolt.org/z/GWWKr8xMd
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