sysgpu: implement select() intrinsic for HLSL, MSL, and SPIR-V

[Crispigt]

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This PR implements the select() intrinsic for HLSL, MSL, and SPIR-V. It also fixes a missing boolean implementation in MSL.

Fixes a panic when compiling shaders using WGSL select().

• HLSL: Mapped to the ternary operator (cond ? true : false).
• MSL: Mapped to metal::select(false, true, cond).
  • Fix: Implemented emitBool for MSL (was previously commented out and not implemented), required for select conditions using literals.
• SPIR-V: Emits OpSelect (OpCode 69).

Left GLSL unimplemented for now as the test suite appears commented out in test.zig, so I figured I wouldn't implement it.

Tested Verified output generation locally for HLSL, MSL, and SPIR-V using a simple test case.
Validated runtime behavior for HLSL and SPIR-V, by rendering a checkered triangle (using select in the fragment shader) on Windows 11 with an AMD Radeon RX 7900 XTX.