Packed structs inside packed struct not initialized properly

[jacobly0]

Extracted from #13480 (comment)

Not sure if this is the same issue @Vexu but I just ran into this:

const std = @import("std");

const Node = packed struct {
    const @"c/d" = packed struct(u40) {
        c: u32 = 0,
        d: u8 = 0,
    };

    const @"a/b" = packed struct(u40) {
        a: u32 = 0,
        b: u8 = 0,
    };

    @"a/b": @"a/b" = .{},
    @"c/d": @"c/d" = .{},

    pub fn init(d: u8) Node {
        return Node{
            // These fields work okay
            .@"a/b" = .{ .a = d, .b = d + 1 },
            // Writing to either of these fields will not work:
            .@"c/d" = .{ .c = d, .d = d + 1 },
        };
    }
};

pub fn main() !void {
    const node = Node.init(9);
    std.debug.print("{any}\n", .{node});
}

; Function Attrs: nounwind uwtable
define internal fastcc i80 @repro.Node.init(i8 %0) unnamed_addr #0 {
  %2 = alloca i80, align 8
  %3 = zext i8 %0 to i32
  store i32 %3, ptr %2, align 8
  %4 = getelementptr inbounds i8, ptr %2, i64 4
  %5 = call fastcc { i8, i1 } @llvm.uadd.with.overflow.i8(i8 %0, i8 1)
  %6 = extractvalue { i8, i1 } %5, 1
  br i1 %6, label %7, label %8

7:                                                ; preds = %1
  call fastcc void @repro.panic(ptr @builtin.panic_messages.integer_overflow__anon_1020, i64 16, ptr null, ptr @0)
  unreachable

8:                                                ; preds = %1
  %9 = extractvalue { i8, i1 } %5, 0
  store i8 %9, ptr %4, align 4
  %10 = zext i8 %0 to i32
  store i32 %10, ptr %2, align 1
  %11 = getelementptr inbounds i8, ptr %2, i64 4
  %12 = call fastcc { i8, i1 } @llvm.uadd.with.overflow.i8(i8 %0, i8 1)
  %13 = extractvalue { i8, i1 } %12, 1
  br i1 %13, label %14, label %15

14:                                               ; preds = %8
  call fastcc void @repro.panic(ptr @builtin.panic_messages.integer_overflow__anon_1020, i64 16, ptr null, ptr @0)
  unreachable

15:                                               ; preds = %8
  %16 = extractvalue { i8, i1 } %12, 0
  store i8 %16, ptr %11, align 1
  %17 = load i80, ptr %2, align 8
  ret i80 %17
}

Note that all stores to c/d occur to a/b instead.

[Pyrolistical]

Seem related to #16581