[hlsl-out] Fix accesses on zero value expressions

[Imberflur]

So that subsequent member access is resolved correctly.

Connections
https://matrix.to/#/!fjjkgHFcwtkREywzfk:matrix.org/$gMvEqmPg59z_xMR6WkUzik82z-UnGezovoXQ4ImVKmM

Description

I was getting HLSL output like:

float4 _expr2703 = float4(_expr2697.x, (float4)0.y, (float4)0.z, (float4)0.w);

Which produced this error:

error X3000: syntax error: unexpected token 'y'

This PR adjusts the output to:

float4 _expr2703 = float4(_expr2697.x, ((float4)0).y, ((float4)0).z, ((float4)0).w);

This is derived from some SPIRV input that I haven't looked through, I tried to reproduce the issue with some simple wgsl like vec2<f32>().x but naga outputs (float)0 for that.

Testing
I compiled my application with this change and stopped getting errors from the hlsl shader compiler.

Checklist

• Run cargo fmt.
• Run cargo clippy. If applicable, add:
  • --target wasm32-unknown-unknown
  • --target wasm32-unknown-emscripten
• Run cargo xtask test to run tests.
• Add change to CHANGELOG.md. See simple instructions inside file.

[Imberflur]

🙁

I'm getting this from the CI:

[ERROR xtask::validate] failed to validate entry point "foo_vert" with profile "vs_5_1": `dxc 'tests\out\hlsl\access.hlsl' -T vs_5_1 -E foo_vert -Wno-parentheses-equality -Zi -Qembed_debug -Od -HV 2018` failed to run; exit code: Some(-2147467259)
stdout:

stderr:
warning: Promoting older shader model profile to 6.0 version.
tests\out\hlsl\access.hlsl:183:41: error: cannot compile this l-value expression yet
    t_1.am = (__mat4x2[2])((float4x2[2])0);
                                        ^

[teoxoy]

That's unfortunate, we might need to generate helper functions for Expression::ZeroValue.

[teoxoy]

Great stuff, thanks!