make else if conditions contain debug stmt info

[andrewrk]

Zig version: 0.10.0-dev.1311+5ea94e771

const std = @import("std");

pub fn main() void {
    var x: i32 = 1234;
    if (x + 1 < 10) {
        x += 1;
    } else if (x / 2 == 0) {
        x += 2;
    }
    var y = x;
    _ = y;
}

Here you can see in a debugging session, the debugger regrettably steps over the x / 2 == 0 expression:

$ ./stage2/bin/zig build-exe test3.zig -fLLVM
$ gdb ./test3
(gdb) break test3.main
Breakpoint 1 at 0x201b24: file test3.zig, line 4.
(gdb) run
Starting program: /home/andy/dev/zig/build-release/test3 

Breakpoint 1, test3.main () at test3.zig:4
4	    var x: i32 = 1234;
(gdb) step
5	    if (x + 1 < 10) {
(gdb) 
10	    var y = x;
(gdb) 

This is because we do not emit a dbg_stmt for it in the ZIR:

        %17 = condbr(%16, {
          %19 = dbg_stmt(4, 9)
          %20 = load(%7) node_offset:6:11
          %21 = typeof(%20) node_offset:6:11
          %22 = add(%20, @Ref.one) node_offset:6:11
          %23 = store(%7, %22)
          %39 = break(%18, @Ref.void_value)
        }, {
          %30 = block({
            %24 = load(%7) node_offset:7:16
            %25 = int(2)
            %26 = div(%24, %25) node_offset:7:18
            %27 = cmp_eq(%26, @Ref.zero) node_offset:7:22
            %28 = as_node(@Ref.bool_type, %27) node_offset:7:22
            %29 = condbr(%28, {
              %31 = dbg_stmt(6, 9)
              %32 = load(%7) node_offset:8:11
              %33 = typeof(%32) node_offset:8:11
              %34 = int(2)
              %35 = add(%32, %34) node_offset:8:11
              %36 = store(%7, %35)
              %37 = break(%30, @Ref.void_value)
            }, {
              %38 = break(%30, @Ref.void_value)
            }) node_offset:7:12
          }) node_offset:7:12

If you ctrl+f to highlight dbg_stmt you can see that one is missing from the else if branch, just before the condition.