pub fn get_block(&self, id: BasicBlockId) -> WeldResult<LLVMBasicBlockRef> {
        self.blocks
            .get(&id)
            .cloned()
            .ok_or_else(|| WeldCompileError::new("Undefined basic block in function codegen"))
    }
Returns the LLVM basic block for a basic block ID in this function.
weld/src/codegen/llvm2/mod.rs