Fixes to reflect that we now consider a SILPhiArgument in a trivially dead block as a phi

With f9861ec we consider SILPhiArgument in a
block with no predecessors as phis, remove assert that we expect non-empt predecessors
to reflect this change.

Author: meg-gupta

lib/SIL/IR/SILArgument.cpp

@@ -175,7 +175,6 @@ bool SILPhiArgument::getIncomingPhiOperands(
     return false;
 
   const auto *parentBlock = getParent();
-  assert(!parentBlock->pred_empty());
 
   unsigned argIndex = getIndex();
   for (auto *predBlock : getParent()->getPredecessorBlocks()) {

lib/SIL/Verifier/SILOwnershipVerifier.cpp

@@ -620,6 +620,10 @@ bool SILValueOwnershipChecker::isSubobjectProjectionWithLifetimeEndingUses(
 bool SILValueOwnershipChecker::
     hasGuaranteedForwardingIncomingPhiOperandsOnZeroOrAllPaths(
         SILPhiArgument *phi) const {
+  // For a phi in a trivially dead block, return true.
+  if (phi->getParentBlock()->pred_empty()) {
+    return true;
+  }
   bool foundGuaranteedForwardingPhiOperand = false;
   bool foundNonGuaranteedForwardingPhiOperand = false;
   phi->visitTransitiveIncomingPhiOperands([&](auto *, auto *operand) -> bool {