implement array subscript for assignment in codegen

test.s

@@ -0,0 +1,90 @@
+.section .text
+.global main
+main:
+push %rbp
+mov %rsp, %rbp
+sub $256, %rsp
+movb $72, -32(%rbp)
+movb $101, -31(%rbp)
+movb $108, -30(%rbp)
+movb $108, -29(%rbp)
+movb $111, -28(%rbp)
+movb $32, -27(%rbp)
+movb $119, -26(%rbp)
+movb $111, -25(%rbp)
+movb $114, -24(%rbp)
+movb $108, -23(%rbp)
+movb $100, -22(%rbp)
+movb $33, -21(%rbp)
+movb $10, -20(%rbp)
+lea -32(%rbp), %rax
+mov %rax, -48(%rbp)
+mov $14, %rax
+mov %rax, -40(%rbp)
+mov -40(%rbp), %rax
+push %rax
+pop %rdi
+call malloc
+mov %rax, -56(%rbp)
+mov -56(%rbp), %rcx
+mov $0, %rax
+push %rax
+mov $13, %rax
+mov %rax, %rdx
+pop %rax
+mov %rdx, %r8
+sub %rax, %r8
+inc %r8
+add %rcx, %rax
+mov %rax, -88(%rbp)
+mov %r8, -80(%rbp)
+lea -88(%rbp), %rax
+mov (%rax), %rcx
+mov 8(%rax), %rdx
+mov %rcx, -72(%rbp)
+mov %rdx, -64(%rbp)
+mov $0, %rax
+mov %rax, -96(%rbp)
+.L0:
+mov -96(%rbp), %rax
+mov %rax, %rcx
+mov -40(%rbp), %rax
+cmp %rax, %rcx
+setl %al
+movzx %al, %rax
+test %rax, %rax
+jz .L1
+mov -72(%rbp), %rcx
+mov -96(%rbp), %rax
+add %rcx, %rax
+push %rax
+mov -48(%rbp), %rcx
+mov -96(%rbp), %rax
+add %rcx, %rax
+movzbl (%rax), %eax
+pop %rcx
+mov %al, (%rcx)
+mov -96(%rbp), %rax
+mov %rax, %rcx
+mov $1, %rax
+add %rcx, %rax
+mov %rax, -96(%rbp)
+jmp .L0
+.L1:
+mov $1, %rax
+push %rax
+mov -72(%rbp), %rax
+push %rax
+mov -64(%rbp), %rax
+push %rax
+pop %rdx
+pop %rsi
+pop %rdi
+call write
+mov $0, %rax
+mov %rbp, %rsp
+pop %rbp
+ret
+mov %rbp, %rsp
+pop %rbp
+ret