[Test] Indicate insts with vals.

If a value is defined by an instruction, it's often more convenient to
specify the instruction by the value.

Author: nate-chandler

include/swift/SIL/ParseTestSpecification.h

@@ -186,7 +186,15 @@ struct Arguments {
   }
   Operand *takeOperand() { return takeInstance<OperandArgument>("operand"); }
   SILInstruction *takeInstruction() {
-    return takeInstance<InstructionArgument>("instruction");
+    auto argument = takeArgument();
+    if (isa<ValueArgument>(argument)) {
+      auto value = cast<ValueArgument>(argument).getValue();
+      auto *definingInstruction = value.getDefiningInstruction();
+      assert(definingInstruction &&
+             "selected instruction via argument value!?");
+      return definingInstruction;
+    }
+    return getInstance<InstructionArgument>("instruction", argument);
   }
   SILArgument *takeBlockArgument() {
     return takeInstance<BlockArgumentArgument>("block argument");