Change translation of implicit throws (issue #1589).

This adds a new invalid instruction that is used for encoding
implicit throws that are emitted by the compiler. This makes it
possible to distinguish such runtime errors from user-provided,
explicit throws.

libsolidity/codegen/CompilerContext.cpp

@@ -215,6 +215,18 @@ CompilerContext& CompilerContext::appendJump(eth::AssemblyItem::JumpType _jumpTy
 	return *this << item;
 }
 
+CompilerContext& CompilerContext::appendInvalid()
+{
+	return *this << Instruction::INVALID;
+}
+
+CompilerContext& CompilerContext::appendConditionalInvalid()
+{
+	eth::AssemblyItem falseTag = appendConditionalJump();
+	eth::AssemblyItem endTag = appendJumpToNew();
+	return *this << falseTag << Instruction::INVALID << endTag;
+}
+
 void CompilerContext::resetVisitedNodes(ASTNode const* _node)
 {
 	stack<ASTNode const*> newStack;