Having more than 32 * 4 scalars in a function signature can cause internal errors with DXC

[ErichDonGubler]

For any of the following $tests:

• webgpu:api,operation,vertex_state,correctness:max_buffers_and_attribs:*
• webgpu:api,operation,vertex_state,correctness:overlapping_attributes:*
• webgpu:compat,api,validation,render_pipeline,vertex_state:maxVertexAttributesVertexIndexInstanceIndex:*

…one provokes internal errors for the DX12 backend when the shaders they produce are compiled. This can be reproduced with an xtask invocation like the following:

$ cargo xtask cts --release -- 'webgpu:api,operation,vertex_state,correctness:max_buffers_and_attribs:*'

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Firefox downstream tracking: bug 1981605

[ErichDonGubler]

There are likely other CTS tests that provoke this, but these are the only ones I found while combing through Firefox crashes.

This happens because we're hitting this assertion here, which limits the "number of scalars" used in a function signature:

https://github.com/FirefoxGraphics/DirectXShaderCompiler/blob/81378be83a1690cac0e727941e1a963518d67c26/lib/HLSL/ComputeViewIdStateBuilder.cpp#L948

I used the following Nushell snippet to reproduce this (which intentionally does not define the $dxc_with_debug_info used therein, see later):

# Nushell FTW! Run these commands from your WGPU checkout.

# Example: first entry from the list in the OP.
let cts_test_path = 'webgpu:api,operation,vertex_state,correctness:max_buffers_and_attribs:*'

let trace_dir = mktemp --tmpdir

DENO_WEBGPU_TRACE=$trace_dir cargo xtask cts --release -- $cts_test_path

let wgsl_in_path = $"($trace_dir)/data1.wgsl"
let hlsl_out_path = $"($trace_dir)/out.hlsl"
cargo run --bin naga -- $wgsl_in_path $hlsl_out_path 

# NOTE: `-T <profile> -E <entry_point>` args. need to be adjusted based on the test.
$dxc_with_debug_info -HV 2018 -Ges -T vs_6_7 -E vsMain $hlsl_out_path

For $dxc_with_debug_info, I used WinDbg with my own DXC build (in this case, https://github.com/FirefoxGraphics/DirectXShaderCompiler/tree/moz-1.8.2502).