zstd: Combine Raw+RLE block fields to reduce buffer and dispatch count

[Jonathan-Weinstein-AMD]

For experimentation, it would be helpful to reduce the number of buffers.

This PR reduces the number of buffers by 5 via combing Raw and RLE block buffers into "RR" buffers. The number of dispatches is also reduced by 5 (though two of those are zero-clear-memsets for lookback buffers, which in some contexts might not be needed anyway).

This mainly works via the added kzstdgpu_RLEBlock_OffsetFlag which the ZstdGpuMemsetMemcpy.hlsl checks for per-block. This flag is encoded in bit 31 (a la lookback buffer style) of the "offset" for RLE blocks, which should be fine since that is already not the actual offset and instead contains the filler byte. The branch becoming non-uniform doesn't really affect performance, and the the shader was slightly restructured so a wave always does one store. Some performance may also be gained by less PrefixSum dispatches, slightly leaner ParseFrames, and in some (maybe contrived) cases where Raw/RLE block destinations are consecutive and have a better memory access pattern.

Noteable changes:

• kzstdgpu_RLEBlock_OffsetFlag.
• zstdgpu_ReferenceStore_Validate_Blocks now validates RLE blocks (as part of "RR" validation); they weren't before (see removed //VALIDATE_BLOCKS(RLE); line). This is handled by zstdgpu_ReferenceStore_Report_Block constructing the "offset" the shader is expected to.
• Avoid nullptr resource barriers with DecompressedSequence{LLen,Offs,MLen} since CMP blocks may not have sequences (such blocks can be useful for testing purposes).

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Testing, used --chk-cpu and --chk-gpu:

• Insects archive.
• Other inputs, which have Huffman-compressed literals.
• Some custom zsts made programmatically, for example:

// A "DegenerateCompressedBlock" is a Compressed_Block with no sequences and non-Huffman compressed literals.
// Useful when just want to test basic things about all blocks types.
ByteVector v;
FrameBuilder fb;
fb.BeginFrame(v);
for (int i = 0; i < 257; ++i)
{
    fb.EmitBlockRLE(v, 'A' + i % 26u, 5); // RHS = len
    fb.EmitBlockRaw(v, "{:}"); // len = 3
    fb.EmitDegenerateCompressedBlockRLE(v, 'a' + i % 26u, 2);
    fb.EmitDegenerateCompressedBlockRaw(v, "-"); // len = 1
}
fb.EndFrame(v);
WriteFile(R"(RR_test_N.txt.zst)", v);