Make coverage expression IDs count up from 0, not down from u32::MAX

Operand types are now tracked explicitly, so there is no need for expression
IDs to avoid counter IDs by descending from `u32::MAX`. Instead they can just
count up from 0, and can be used directly as indices when necessary.

compiler/rustc_middle/src/ty/structural_impls.rs

@@ -471,8 +471,7 @@ TrivialTypeTraversalAndLiftImpls! {
     ::rustc_target::asm::InlineAsmRegOrRegClass,
     ::rustc_target::spec::abi::Abi,
     crate::mir::coverage::CounterValueReference,
-    crate::mir::coverage::InjectedExpressionId,
-    crate::mir::coverage::InjectedExpressionIndex,
+    crate::mir::coverage::ExpressionId,
     crate::mir::coverage::MappedExpressionIndex,
     crate::mir::Local,
     crate::mir::Promoted,