Rollup merge of #125106 - Zalathar:expressions, r=davidtwco

coverage: Memoize and simplify counter expressions

When creating coverage counter expressions as part of coverage instrumentation, we often end up creating obviously-redundant expressions like `c1 + (c0 - c1)`, which is equivalent to just `c0`.

To avoid doing so, this PR checks when we would create an expression matching one of 5 patterns, and uses the simplified form instead:
- `(a - b) + b` → `a`.
- `(a + b) - b` → `a`.
- `(a + b) - a` → `b`.
- `a + (b - a)` → `b`.
- `a - (a - b)` → `b`.

Of all the different ways to combine 3 operands and 2 operators, these are the patterns that allow simplification.

(Some of those patterns currently don't occur in practice, but are included anyway for completeness, to avoid having to add them later as branch coverage and MC/DC coverage support expands.)

---

This PR also adds memoization for newly-created (or newly-simplified) counter expressions, to avoid creating duplicates.

This currently makes no difference to the final mappings, but is expected to be useful for MC/DC coverage of match expressions, as proposed by https://github.com/rust-lang/rust/pull/124278#issuecomment-2106754753.

compiler/rustc_middle/src/mir/coverage.rs

@@ -181,8 +181,8 @@ impl Debug for CodeRegion {
     }
 }
 
-#[derive(Copy, Clone, Debug, PartialEq, TyEncodable, TyDecodable, Hash, HashStable)]
-#[derive(TypeFoldable, TypeVisitable)]
+#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash, HashStable)]
+#[derive(TyEncodable, TyDecodable, TypeFoldable, TypeVisitable)]
 pub enum Op {
     Subtract,
     Add,