[vm] Update assembler to access AbstractType::flags_ as Uint32.

In d592882f, the old uint8_t `type_state_` and `nullability_` fields
in subclasses of `AbstractType` were merged into a single uint32_t
`flags_` field in `AbstractType` itself.

Accessing the nullability information was abstracted into two methods:

* `LoadAbstractTypeNullability`
* `CompareAbstractTypeNullabilityWith`

However, the code from these methods were based off the old
`CompareTypeNullabilityWith` and `CompareFunctionTypeNullabilityWith`
methods that used unsigned byte loads to access the old field, and kept
that even though the new field is now a 32-bit unsigned integer. This
works for now, because the nullability portion of the `flags_` field
happens to be the lowest two bits, but might lead to confusion if
someone attempts to write similar code for accessing the other parts of
the `flags_` field without realizing it's larger than 8 bits.

Thus, this CL does the following:

* Unifies the implementation of both methods across architectures.
* Adds a native slot definition for the `AbstractType::flags_` field
  and uses `LoadFromSlot` to load the `flags_` field instead. Other
  places where the `AbstractType::flags_` field is accessed from the
  assembler are also changed to use `LoadFromSlot` appropriately.

TEST=The existing suite of tests are sufficient.

Change-Id: Ie578e4dddce810223ca9e767403136cb40d340f3
Cq-Include-Trybots: luci.dart.try:vm-aot-linux-release-x64-try,vm-aot-linux-release-simarm_x64-try,vm-aot-linux-release-simarm64-try,vm-aot-linux-debug-simarm_x64-try,vm-aot-linux-debug-simriscv64-try,vm-aot-mac-release-arm64-try,vm-aot-mac-release-x64-try,vm-aot-linux-debug-x64c-try,vm-kernel-precomp-linux-release-simarm-try,vm-linux-release-x64-try,vm-linux-release-simarm64-try,vm-linux-release-simarm-try,vm-linux-release-ia32-try,vm-linux-debug-simriscv64-try
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/302187
Reviewed-by: Daco Harkes <dacoharkes@google.com>
Commit-Queue: Tess Strickland <sstrickl@google.com>
Reviewed-by: Ryan Macnak <rmacnak@google.com>