`extern_weak` globals may be nullptr

[workingjubilee]

Alive2 asserts this transformation is correct: https://alive2.llvm.org/ce/z/YX8gMP

@GLIBC_PRIVATE = extern_weak global {}

define noundef zeroext i1 @src() unnamed_addr {
start:
  %_ptr = ptrtoint ptr @GLIBC_PRIVATE to i64
  %_0 = icmp eq i64 %_ptr, 0
  ret i1 %_0
}

define noundef zeroext i1 @tgt() unnamed_addr {
start:
  ret i1 false
}

However, I'm told by llvm/llvm-project#133621 that this is in fact incorrect specifically for the case of extern_weak linkage. This is a fairly subtle bug, but may be because Alive2 is discarding the linkage instead of examining it.

[nunoplopes]

Linkage is currently ignored by Alive.