Internal error during optimization for OpDecorateString with multiple literals

The following GLSL fragment shader fails to compile to SPIR-V when optimizations are turned on:

```
#extension GL_EXT_spirv_intrinsics : require

spirv_decorate_string(5635, "string1", "string2") layout(location = 0) out float test;

void main() {
test = 0.0;
}
```

It seems that the optimizer does not handle OpDecorateString opcodes that have multiple string literals correctly.
```
shaderc: internal error: compilation succeeded but failed to optimize: Invalid instruction OpDecorateString starting at word 184: expected no more operands after 5 words, but stated word count is 7.
```

Without optimizations, the shader compiles correctly, and both the "string1" and "string2" literals are correctly included in the resulting OpDecorateString, so clearly the error is invalid; the OpDecorateString instruction IS valid and correct.