implement storing vector elements via runtime index

test/compile_errors.zig

@@ -26,6 +26,23 @@ pub fn addCases(cases: *tests.CompileErrorContext) void {
 
     cases.add(
         "dereference vector pointer with unknown runtime index",
+        "store vector pointer with unknown runtime index",
+        \\export fn entry() void {
+        \\    var v: @Vector(4, i32) = [_]i32{ 1, 5, 3, undefined };
+        \\
+        \\    var i: u32 = 0;
+        \\    storev(&v[i], 42);
+        \\}
+        \\
+        \\fn storev(ptr: var, val: i32) void {
+        \\    ptr.* = val;
+        \\}
+    ,
+        "tmp.zig:9:8: error: unable to determine vector element index of type '*align(16:0:4:?) i32",
+    );
+
+    cases.add(
+        "load vector pointer with unknown runtime index",
         \\export fn entry() void {
         \\    var v: @Vector(4, i32) = [_]i32{ 1, 5, 3, undefined };
         \\