[SPIRV] "casting to type 'RWByteAddressBuffer' unimplemented" when using ternary/phi with local resources

When compiling HLSL that uses a ternary expression to select between two global resources and assign to a local resource variable, DXC's SPIRV backend emits:

error: casting to type 'RWByteAddressBuffer' unimplemented

Repro:

RWByteAddressBuffer gBuf0 : register(u0); 
RWByteAddressBuffer gBuf1 : register(u1);

[numthreads(1,1,1)] 
void main() {  
  RWByteAddressBuffer buf = (true ? gBuf0 : gBuf1); 
  buf.Store(0, 42);
}

Command:

dxc.exe -spirv -fspv-target-env=vulkan1.3 -T cs_6_0 -Fo out.spv repro.hlsl

This also reproduces with nested if/else phi patterns (e.g. buf = false ? gBuf0 : gBuf1 inside an if block). DXC DXIL and Clang DXIL both compile this successfully. DXC version: main (built from source).

Start by reproducing the failure with the provided HLSL input and dxc.exe SPIR-V command, then trace the SPIR-V backend handling of ternary or phi-selected local resources. Done means both the ternary and nested if/else cases compile successfully and the resulting shader preserves the buffer Store operation.