[diagnose-unreachable] Constant fold simple switch_enum_addr to eliminate more unreachable code.

This patch comes out of my reading some generic code using .none in transparent
functions to conditionally compile out code at -Onone. Sadly, before this the
dead code in question wouldn't be compiled out unless the protocol was
constrained to be a class protocol.

I added a test that validates that this conditional compilation property can be
relied on in -Onone code in both cases.

Author: gottesmm

include/swift/SIL/Projection.h

@@ -377,15 +377,15 @@ class Projection {
 
   /// Returns true if this instruction projects from an address type to an
   /// address subtype.
-  static SingleValueInstruction *isAddressProjection(SILValue V) {
-    switch (V->getKind()) {
+  static SingleValueInstruction *isAddressProjection(SILValue v) {
+    switch (v->getKind()) {
     default:
       return nullptr;
     case ValueKind::IndexAddrInst: {
-      auto I = cast<IndexAddrInst>(V);
-      unsigned Scalar;
-      if (getIntegerIndex(I->getIndex(), Scalar))
-        return I;
+      auto *i = cast<IndexAddrInst>(v);
+      unsigned scalar;
+      if (getIntegerIndex(i->getIndex(), scalar))
+        return i;
       return nullptr;
     }
     case ValueKind::StructElementAddrInst:
@@ -394,10 +394,17 @@ class Projection {
     case ValueKind::ProjectBoxInst:
     case ValueKind::TupleElementAddrInst:
     case ValueKind::UncheckedTakeEnumDataAddrInst:
-      return cast<SingleValueInstruction>(V);
+      return cast<SingleValueInstruction>(v);
     }
   }
 
+  static SingleValueInstruction *isAddressProjection(SILInstruction *i) {
+    auto *svi = dyn_cast<SingleValueInstruction>(i);
+    if (!svi)
+      return nullptr;
+    return isAddressProjection(SILValue(svi));
+  }
+
   /// Returns true if this instruction projects from an object type to an object
   /// subtype.
   static SingleValueInstruction *isObjectProjection(SILValue V) {