HLSL: structured buffer getter generates broken spirv

Hello,

I'm using a small helper function to retrieve the exposure buffer to read.

```

struct AutoExposureInfos
{
float luminanceFactor;
float targetLuminance;
float minLuminanceLDR;
float maxLuminanceLDR;
float mgLuminanceLDR;
float ev;
float fStop;
uint maxHistogramValue;
};

inline AutoExposureInfos GetAutoExposureParameters( StructuredBuffer buffer )
{
return buffer[0];
}
```

Example:
```
AutoExposureInfos currentExposure = GetAutoExposureParameters( AutoExposureBuffer );
float currentEV = computeEV100FromAvgLuminance( currentExposure.luminanceFactor );
```
It seems to generate broken bytecode, making my vkCreateComputePipelines call to crash in some random assembly code.

I have tried to hardcode the buffer index in the shader ( AutoExposureBuffer[0] ) and it seems to solve the problem.

[Shader Disassembly](https://github.com/KhronosGroup/glslang/files/3030754/FinalPost.vkps.txt)

Start by reproducing the HLSL structured-buffer getter example and comparing its generated shader disassembly with the version that uses AutoExposureBuffer[0] directly. Inspect the generated SPIR-V around the helper's buffer access, then verify that the corrected output can be consumed by vkCreateComputePipelines without crashing.

Written by the indexing model from the issue text.