AArch64: Safely handle the incoming sret call argument.

This adds a safe interface to the machine independent InputArg struct
for accessing the index of the original (IR-level) argument. When a
non-native return type is lowered, we generate the hidden
machine-level sret argument on-the-fly. Before this fix, we were
representing this argument as OrigArgIndex == 0, which is an outright
lie. In particular this crashed in the AArch64 backend where we
actually try to access the type of the original argument.

Now we use a sentinel value for machine arguments that have no
original argument index. AArch64, ARM, Mips, and PPC now check for this
case before accessing the original argument.

Fixes <rdar://19792160> Null pointer assertion in AArch64TargetLowering

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@229413 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Target/TargetCallingConv.h

@@ -134,6 +134,8 @@ namespace ISD {
 
     /// Index original Function's argument.
     unsigned OrigArgIndex;
+    /// Sentinel value for implicit machine-level input arguments.
+    static const unsigned NoArgIndex = UINT_MAX;
 
     /// Offset in bytes of current input value relative to the beginning of
     /// original argument. E.g. if argument was splitted into four 32 bit
@@ -147,6 +149,15 @@ namespace ISD {
       VT = vt.getSimpleVT();
       ArgVT = argvt;
     }
+
+    bool isOrigArg() const {
+      return OrigArgIndex != NoArgIndex;
+    }
+
+    unsigned getOrigArgIndex() const {
+      assert(OrigArgIndex != NoArgIndex && "Implicit machine-level argument");
+      return OrigArgIndex;
+    }
   };
 
   /// OutputArg - This struct carries flags and a value for a