Extend quir::SwitchOp lowering to correctly lower results. (#153)

This patch extends the quir::SwitchOp lowering to LLVM IR to correctly
handle results. These changes are based on the support added to
scf::IndexSwitchOp operation, which also supports switch ops with
results.

Support for lowering the scf::IndexSwitchOp was added to MLIR in commit
91effec85 (llvm/llvm-project@91effec85).

Author: kitbarton

lib/Conversion/QUIRToStandard/SwitchOpLowering.cpp

@@ -53,26 +53,19 @@ struct SwitchOpLowering : public OpRewritePattern<SwitchOp> {
 LogicalResult
 SwitchOpLowering::matchAndRewrite(SwitchOp switchOp,
                                   PatternRewriter &rewriter) const {
-  assert(switchOp.resultTypes().size() == 0 &&
-         "switch ops with result values are not supported");
-
   auto loc = switchOp.getLoc();
 
   // Start by splitting the block containing the 'quir.switch' into parts.
   // The part before will contain the condition, the part after will be the
   // continuation point.
   auto *condBlock = rewriter.getInsertionBlock();
   auto opPosition = rewriter.getInsertionPoint();
-  auto *remainingOpsBlock = rewriter.splitBlock(condBlock, opPosition);
-  Block *continueBlock;
-  if (switchOp.getNumResults() == 0) {
-    continueBlock = remainingOpsBlock;
-  } else {
-    continueBlock =
-        rewriter.createBlock(remainingOpsBlock, switchOp.getResultTypes());
-    rewriter.create<BranchOp>(loc, remainingOpsBlock);
-  }
-
+  Block *continueBlock = rewriter.splitBlock(condBlock, opPosition);
+  SmallVector<Value> results;
+  results.reserve(switchOp.getNumResults());
+  for (Type resultType : switchOp.getResultTypes())
+    results.push_back(
+        continueBlock->addArgument(resultType, switchOp.getLoc()));
   // Move blocks from the "default" region to the region containing
   // 'quir.switch', place it before the continuation block, and branch to it.
   auto &defaultRegion = switchOp.defaultRegion();