Add a helper function for emitting a switch_enum that breaks down an Optional.

Author: rjmccall

include/swift/SIL/SILBuilder.h

@@ -2706,6 +2706,17 @@ class SILBuilder {
         CaseCounts, DefaultCount, forwardingOwnershipKind));
   }
 
+  /// A convenience function to decompose a scalar optional value with a
+  /// switch_enum.  Returns the object value, which will only be valid
+  /// in `someBB`.  Don't forget to switch insertion blocks after
+  /// calling this.
+  SILPhiArgument *createSwitchOptional(
+      SILLocation loc, SILValue operand,
+      SILBasicBlock *someBB, SILBasicBlock *noneBB,
+      ValueOwnershipKind forwardingOwnershipKind,
+      ProfileCounter someCount = ProfileCounter(),
+      ProfileCounter noneCount = ProfileCounter());
+
   SwitchEnumAddrInst *createSwitchEnumAddr(
       SILLocation Loc, SILValue Operand, SILBasicBlock *DefaultBB,
       ArrayRef<std::pair<EnumElementDecl *, SILBasicBlock *>> CaseBBs,

lib/SIL/IR/SILBuilder.cpp

@@ -681,6 +681,25 @@ void SILBuilder::emitScopedBorrowOperation(SILLocation loc, SILValue original,
     createEndBorrow(loc, value);
 }
 
+SILPhiArgument *SILBuilder::createSwitchOptional(
+                                SILLocation loc, SILValue operand,
+                                SILBasicBlock *someBB, SILBasicBlock *noneBB,
+                                ValueOwnershipKind forwardingOwnershipKind,
+                                ProfileCounter someCount,
+                                ProfileCounter noneCount) {
+  ProfileCounter counts[] = {someCount, noneCount};
+  std::optional<ArrayRef<ProfileCounter>> countsArg = std::nullopt;
+  if (someCount || noneCount) countsArg = counts;
+
+  auto &ctx = getASTContext();
+  auto sei = createSwitchEnum(loc, operand, /*default*/ nullptr,
+                              {{ctx.getOptionalSomeDecl(), someBB},
+                               {ctx.getOptionalNoneDecl(), noneBB}},
+                              countsArg, /*default*/ProfileCounter(),
+                              forwardingOwnershipKind);
+  return sei->createResult(someBB, operand->getType().unwrapOptionalType());
+}
+
 /// Attempt to propagate ownership from \p operand to the returned forwarding
 /// ownership where the forwarded value has type \p targetType. If this fails,
 /// return Owned forwarding ownership instead.