[vm/test] Adds regression test for movb on x64.
[vm/test] Adds regression test for movb on x64.

Fixed in: https://dart-review.googlesource.com/c/sdk/+/237690

Adds MoveByteRunTest, testing that we emit the correct assembly
for movb so that we are able to run it.

This includes a fix for the issue the cause this same change to
previously get reverted:
https://github.com/dart-lang/sdk/issues/48661
Namely, the test now saves RDI since it is non-volatile on Windows.

TEST=Adds cc/vm/MoveByteRunTest
Cq-Include-Trybots: luci.dart.try:dart-sdk-win-try,vm-kernel-precomp-win-debug-x64c-try,vm-kernel-win-debug-x64-try
Change-Id: I97cad1367a5d33cfca36a880260e1771052af003
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/238701
Reviewed-by: Martin Kustermann <a35dd5b79fac84b2bb3b026a16b8becc55af5961@google.com>
Auto-Submit: Clement Skau <495593248b6158163719b90dde8357b589c8e5d0@google.com>
Commit-Queue: Clement Skau <495593248b6158163719b90dde8357b589c8e5d0@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk