Some statements (assignment?) have no line representation in debug.pc2line_buf table and don't work with `find_line_num`

[chmeyer-ms]

I've been using find_line_num to get line numbers from opcodes. I noticed that certain lines don't have the correct line number in the table.

For example, given this snippet:

let str1 = 'my string';
let str2 = 'another str';
let str3 = '3 string';
let str4 = str1 + str2 + str3;

The opcodes (from js_dump_function_bytecode) are:

opcodes:
      push_this
      if_false8 4
      return_undef
  4:  push_atom_value "my string"
      put_var_ref0 0: str1
      push_atom_value "another str"
      put_var_ref1 1: str2
      push_atom_value "3 string"
      put_var_ref2 2: str3
      get_var_ref_check 0: str1
      get_var_ref_check 1: str2
      add
      get_var_ref_check 2: str3
      add
      put_var_ref3 3: str4
      undefined
      return_async

And the PC to Line output (from 'dump_pc2line') is:

 PC  LINE   COL
  -     1     1
 22     4    12 <-- no 2 or 3
 25     4    19
 28     4    17
 29     4    26
 32     4    24

No line 2 or 3.

But if the snippet is altered to do more than simple assignment like so:

let str1 = 'my' + 'string';
let str2 = 'another' + 'str';
let str3 = '3' + 'string';
let str4 = str1 + str2 + str3;

The opcodes are:

opcodes:
    push_this
    if_false8 4
    return_undef
4:  push_atom_value my
    push_atom_value string
    add
    put_var_ref0 0: str1
    push_atom_value another
    push_atom_value str
    add
    put_var_ref1 1: str2
    push_const8 0: "3"
    push_atom_value string
    add
    put_var_ref2 2: str3
    get_var_ref_check 0: str1
    get_var_ref_check 1: str2
    add
    get_var_ref_check 2: str3
    add
    put_var_ref3 3: str4
    undefined
    return_async

The PC to Line output is:

PC  LINE   COL
 -     1     1
14     1    17
26     2    22 <-- line 2 and 3 are back
35     3    16
37     4    12
40     4    19
43     4    17
44     4    26
47     4    24

This is problematic for our custom debugging solution.

I haven't verified the bisect, but it worked in the 2024-01-13 release and I suspect it was changed with this commit that added column support.