Cleaning up write barrier code

[SingleAccretion]

I was investigating some diffs from deleting the last of CLS_VAR (on ARM), and the majority had to do with how we select write barriers for stores. I found the relevant code that does that somewhat confusing, so, two changes:

Write barrier code cleanup

1) Delete leftovers from legacy codegen, including the "tgt" naming.
2) Do not second-guess the write barrier form when selecting a helper,
   always return "known" WBF from "gcIsWriteBarrierCandidate" instead.

Delete GTF_IND_TGT_ANYWHERE

And create GTF_IND_TGT_HEAP instead, its logical opposite.

For reference, the situation before this change was as follows:

// Type of the ("simplified"/"base") address:
//
//  - TYP_I_IMPL: checked write barrier
//  - TYP_REF:    unchecked write barrier
//  - TYP_BYREF:
//    1) No flags:     unchecked write barrier
//    2) TGT_ANYWHERE: checked write barrier

The TYP_BYREF case will now look like this:
//
//    1) No flags: checked write barrier
//    2) TGT_HEAP: unchecked write barrier

The main benefit is that we get the default state (no flags) to
be the conservatively correct one.

No diffs are expected.

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Issue Details

---

I was investigating some diffs from deleting the last of CLS_VAR (on ARM), and the majority had to do with how we select write barriers for stores. I found the relevant code that does that somewhat confusing, so, two changes:

Write barrier code cleanup

1) Delete leftovers from legacy codegen, including the "tgt" naming.
2) Do not second-guess the write barrier form when selecting a helper,
   always return "known" WBF from "gcIsWriteBarrierCandidate" instead.

Delete GTF_IND_TGT_ANYWHERE

And create GTF_IND_TGT_HEAP instead, its logical opposite.

For reference, the situation before this change was as follows:

// Type of the ("simplified"/"base") address:
//
//  - TYP_I_IMPL: checked write barrier
//  - TYP_REF:    unchecked write barrier
//  - TYP_BYREF:
//    1) No flags:     unchecked write barrier
//    2) TGT_ANYWHERE: checked write barrier

The TYP_BYREF case will now look like this:
//
//    1) No flags: checked write barrier
//    2) TGT_HEAP: unchecked write barrier

The main benefit is that we get the default state (no flags) to
be the conservatively correct one.

No diffs are expected.

Author:	SingleAccretion
Assignees:	-
Labels:	area-CodeGen-coreclr, community-contribution
Milestone:	-

[SingleAccretion]

The new FEATURE_COUNT_GC_WRITE_BARRIERS code is a best-effort construction based on what I thought was "reasonable", I did not test it.

[SingleAccretion]

No diffs, but there is one seemingly pre-existing related SPMI failure:

[14:39:00] ERROR: Exception thrown: SuperPMI assertion 'srcBuffer != nullptr' failed

[14:39:00] ERROR: Exception thrown: SuperPMI assertion 'srcBuffer != nullptr' failed

[14:39:00] ERROR: Method 39626 of size 41 failed to load and compile correctly by JIT2.

[14:39:00] ERROR: main method 39626 of size 41 failed to load and compile correctly.

[SingleAccretion]

@dotnet/jit-contrib