[SIL] Deoptionalize asFullApplySite.

The FullApplySite itself can already be null, and indeed this function
was producing `llvm::Some(FullApplySite())` when called on ApplySite
instances which weren't actually full.

Author: nate-chandler

include/swift/SIL/ApplySite.h

@@ -541,9 +541,9 @@ class ApplySite {
 
   void dump() const LLVM_ATTRIBUTE_USED { getInstruction()->dump(); }
 
-  /// Attempt to cast this apply site to a full apply site, returning None on
-  /// failure.
-  llvm::Optional<FullApplySite> asFullApplySite() const;
+  /// Form a FullApplySite.  Note that it will be null if this apply site is not
+  /// in fact a FullApplySite.
+  FullApplySite asFullApplySite() const;
 };
 
 //===----------------------------------------------------------------------===//
@@ -818,15 +818,15 @@ template <> struct DenseMapInfo<::swift::FullApplySite> {
 
 namespace swift {
 
-inline llvm::Optional<FullApplySite> ApplySite::asFullApplySite() const {
+inline FullApplySite ApplySite::asFullApplySite() const {
   return FullApplySite::isa(getInstruction());
 }
 
 inline bool ApplySite::isIndirectResultOperand(const Operand &op) const {
   auto fas = asFullApplySite();
   if (!fas)
     return false;
-  return fas->isIndirectResultOperand(op);
+  return fas.isIndirectResultOperand(op);
 }
 
 } // namespace swift