Sema: use elemPtrOneLayerOnly in zirMemCpy

Closes #15633

Author: Vexu

src/Sema.zig

@@ -22138,7 +22138,7 @@ fn zirMemcpy(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!void
             const len = try sema.usizeCast(block, dest_src, len_u64);
             for (0..len) |i| {
                 const elem_index = try sema.addIntUnsigned(Type.usize, i);
-                const dest_elem_ptr = try sema.elemPtr(
+                const dest_elem_ptr = try sema.elemPtrOneLayerOnly(
                     block,
                     src,
                     dest_ptr,
@@ -22147,7 +22147,7 @@ fn zirMemcpy(sema: *Sema, block: *Block, inst: Zir.Inst.Index) CompileError!void
                     true, // init
                     false, // oob_safety
                 );
-                const src_elem_ptr = try sema.elemPtr(
+                const src_elem_ptr = try sema.elemPtrOneLayerOnly(
                     block,
                     src,
                     src_ptr,

test/behavior/memcpy.zig

@@ -65,3 +65,16 @@ fn testMemcpyDestManyPtr() !void {
     try expect(buf[98] == 'l');
     try expect(buf[99] == 'o');
 }
+
+comptime {
+    const S = struct {
+        buffer: [8]u8 = undefined,
+        fn set(self: *@This(), items: []const u8) void {
+            @memcpy(self.buffer[0..items.len], items);
+        }
+    };
+
+    var s = S{};
+    s.set("hello");
+    if (!std.mem.eql(u8, s.buffer[0..5], "hello")) @compileError("bad");
+}