WASM_X64: Support if else

[ubaidsk]

This PR adds support for if-else statements in the wasm_x64 backend.

[ubaidsk]

I initially attempted to support the idea of using unique integers shared at #1264 (comment).

For example using:

void visit_If() {
        unique_idx++;
        std::string label = std::to_string(unique_idx);
        m_a.asm_pop_r64(X64Reg::rax); // now `rax` contains the logical value (true = 1, false = 0) of the if condition
        m_a.asm_cmp_r64_imm8(X64Reg::rax, 1);
        m_a.asm_je_label(".then_" + label);
        m_a.asm_jmp_label(".else_" + label);
        m_a.add_label(".then_" + label);
        {
            decode_instructions();
        }
        m_a.add_label(".endif_" + label);
        unique_idx--;
    }

    void visit_Else() {
        std::string label = std::to_string(unique_idx);
        m_a.asm_jmp_label(".endif_" + label);
        m_a.add_label(".else_" + label);
    }

But it happens that the if-else statements within the same scope have the same ids as follows:

if { ---- index 0
    if { --- index 1
        if { --- index 2

        } else { --- index 2

        }        
    } else { --- index 1

    }
   
    if { -- index 1
   
    } else { --- index 1
    
    }
} 

Thus, I used the stack approach similar to the x86 backend.

[ubaidsk]

Thank you for the approval. I am adding it to auto-merge as it seems approved.