[TableGen] Remove unnecessary TypeSetByHwMode -> ValueTypeByHwMode -> TypeSetByHwMode conversions in getPatternSize

[RKSimon]

Summary

I noticed this during profiling of tablegen (PR28222) that we were calling Child->getType(0) which creates a ValueTypeByHwMode on the fly from the requested internal TypeSetByHwMode type and returns it by value, we then treat it as a TypeSetByHwMode reference which involves constructing a new TypeSetByHwMode on the stack with a large amount of std::map iterating/copying all along the way.

I am not an expert on tablegen so please check my working, but AFAICT this is all unnecessary and we should be calling Child->getExtType(0) which returns the original TypeSetByHwMode by reference.

This gives me a 90sec reduction in msvc debug builds of x86 -gen-dag-isel.

[zturner]

This looks great. I've tried to reduce the MSVC debug build time as well,
but I didn't find this particular problem. I can't comment on the
correctness since I don't know much about how tablegen is implemented, but
I just wanted to say that this kind of thing is very appreciated and I hope
you continue doing more of it :)

llvm/trunk/utils/TableGen/CodeGenDAGPatterns.cpp

static unsigned getPatternSize(const TreePatternNode *P,
   // complexity of this node.
   if (!P->getPredicateFns().empty())
     ++Size;
 
   // Count children in the count if they are also nodes.
   for (unsigned i = 0, e = P->getNumChildren(); i != e; ++i) {
     const TreePatternNode *Child = P->getChild(i);
     if (!Child->isLeaf() && Child->getNumTypes()) {
-      const TypeSetByHwMode &T0 = Child->getType(0);
+      const TypeSetByHwMode &T0 = Child->getExtType(0);
       // At this point, all variable type sets should be simple, i.e. only
       // have a default mode.
       if (T0.getMachineValueType() != MVT::Other) {
         Size += getPatternSize(Child, CGP);
         continue;
       }
     }
     if (Child->isLeaf()) {