pub unsafe fn filled(&mut self, builder: LLVMBuilderRef, value: LLVMValueRef) -> LLVMValueRef {
        let filled_pointer = LLVMBuildStructGEP(builder, value, FILLED_INDEX, c_str!(""));
        let filled = self.load(builder, filled_pointer).unwrap();
        LLVMBuildICmp(
            builder,
            LLVMIntNE,
            filled,
            self.i8(0),
            c_str!("slot.filled"),
        )
    }
Return whether the provided slot is filled as an `i1`.

The slot is filled if the byte value of the filled field is non-zero.

The slot should be a pointer.
weld/src/codegen/llvm2/dict.rs