[vm/compiler] Recognize and propagate "value can be sentinel" property This change adds can_be_sentinel() flag to CompileType and prevents unboxing of phis which can be sentinel. This flag means that set of values can potentially contain Object::sentinel() which is used as a marker for the uninitialized value of late variables. TEST=runtime/tests/vm/dart/regress_46141_test.dart Fixes https://github.com/dart-lang/sdk/issues/46141 Change-Id: I32f19488f54c6f69932584ecec3094e3b78cc0d0 Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/201600 Reviewed-by: Slava Egorov <vegorov@google.com> Commit-Queue: Alexander Markov <alexmarkov@google.com>
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