[vm] Fix UB in materialization of Float32List
[vm] Fix UB in materialization of Float32List

This is the follow-up to https://dart-review.googlesource.com/c/sdk/+/193831.
This change fixes undefined behavior corresponding to read from
the member of the union that wasn't most recently written.

TEST=ci

Change-Id: I5a74a75f8dca3526068880bfd3a848b611d6cbf2
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/193887
Reviewed-by: Vyacheslav Egorov <72c81f38df725d8f1832acb9b5bb99563fc2f1b9@google.com>
Commit-Queue: Alexander Markov <239e774fd29abc1267878f57afe1a8a03e0ca887@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