[clang] [CIR] Implement switch case simplify (PR #140649)

[Andy Kaylor via cfe-commits]

================
@@ -159,6 +159,107 @@ struct SimplifySelect : public OpRewritePattern<SelectOp> {
   }
 };
 
+/// Simplify `cir.switch` operations by folding cascading cases
+/// into a single `cir.case` with the `anyof` kind.
+///
+/// This pattern identifies cascading cases within a `cir.switch` operation.
+/// Cascading cases are defined as consecutive `cir.case` operations of kind
+/// `equal`, each containing a single `cir.yield` operation in their body.
+///
+/// The pattern merges these cascading cases into a single `cir.case` operation
+/// with kind `anyof`, aggregating all the case values.
+///
+/// The merging process continues until a `cir.case` with a different body
+/// (e.g., containing `cir.break` or compound stmt) is encountered, which
+/// breaks the chain.
+///
+/// Example:
+///
+/// Before:
+///   cir.case equal, [#cir.int<0> : !s32i] {
+///     cir.yield
+///   }
+///   cir.case equal, [#cir.int<1> : !s32i] {
+///     cir.yield
+///   }
+///   cir.case equal, [#cir.int<2> : !s32i] {
+///     cir.break
+///   }
+///
+/// After applying SimplifySwitch:
+///   cir.case anyof, [#cir.int<0> : !s32i, #cir.int<1> : !s32i, #cir.int<2> :
+///   !s32i] {
+///     cir.break
+///   }
+struct SimplifySwitch : public OpRewritePattern<SwitchOp> {
+  using OpRewritePattern<SwitchOp>::OpRewritePattern;
+  LogicalResult matchAndRewrite(SwitchOp op,
+                                PatternRewriter &rewriter) const override {
+
+    LogicalResult changed = mlir::failure();
+    llvm::SmallVector<CaseOp, 8> cases;
----------------
andykaylor wrote:

```suggestion
    SmallVector<CaseOp, 8> cases;
```

https://github.com/llvm/llvm-project/pull/140649