Rollup merge of #127758 - Zalathar:expression-used, r=oli-obk

coverage: Restrict `ExpressionUsed` simplification to `Code` mappings

In the future, branch and MC/DC mappings might have expressions that don't correspond to any single point in the control-flow graph. That makes it trickier to keep track of which expressions should expect an `ExpressionUsed` node.

We therefore sidestep that complexity by only performing `ExpressionUsed` simplification for expressions associated directly with ordinary `Code` mappings.

(This simplification step is inherited from the original coverage implementation, which only supported `Code` mappings anyway, so there's no particular reason to extend it to other kinds of mappings unless we specifically choose to.)

Relevant to:
- #124154
- #126677
- #124278

```@rustbot``` label +A-code-coverage

Author: matthiaskrgr