[vm] Relax AssertAssignable to allow dst_type to be non-constant.

Previously, AssertAssignable instructions could only be created with a
compile-time AbstractType value for dst_type. Relax this so that
AssertAssignable instructions can be created with a possibly
non-constant Value instead.

This is only an IL-level change for the instruction. Code generation for
AssertAssignable still requires the dst_type Value to be bound to a
constant and is otherwise unchanged from before, thus this should have
no impact on size or speed of generated code. (Generating the
non-constant case will be done in follow-up work.)

Bug: https://github.com/dart-lang/sdk/issues/40813

Cq-Include-Trybots: luci.dart.try:vm-kernel-precomp-linux-release-x64-try,vm-kernel-precomp-linux-product-x64-try,vm-kernel-precomp-linux-debug-x64-try,vm-kernel-precomp-linux-release-simarm-try,vm-kernel-precomp-linux-release-simarm_x64-try,vm-kernel-precomp-linux-release-simarm64-try,vm-dartkb-linux-release-simarm64-try,vm-dartkb-linux-release-x64-try
Bug: https://github.com/dart-lang/sdk/issues/40813
Change-Id: I1f984039d8c2695c66161a69a3d80cfbe7110f19
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/146801
Commit-Queue: Tess Strickland <sstrickl@google.com>
Reviewed-by: Martin Kustermann <kustermann@google.com>
Reviewed-by: Vyacheslav Egorov <vegorov@google.com>