HLSL_IR_STORE and HLSL_IR_RESOURCE_STORE exist though. I don't think you can use those as HLSL_IR_EXPR sources (right?), but that might not necessarily be obvious. It does touch a bit on what I think the issue with "MOVC" is as well though; "MOVC" sounds suspiciously like something that might be called a conditional store operation.
Yes, those have a side effect, but they don't yield a value.
Naming is hard and I don't think that we're going to have a perfect name here, which is why we need this documentation.