Add a function to get the largest address space pointer size

[arsenm]

Summary

Some users of DataLayout::getPointerSize don't use it in an exact way, and aren't really using it for the pointer size. For example MemCpyOptimizer uses it as a guess for the maximum GPR width.

[meadori]

I don't have much to say about the method itself, but usually patches that introduce a new function/method have diffs to actually use that method as well. Does 'getMaxPointerSize' have any specific uses?

[arsenm]

I think it's usually the opposite, you add the function and then the uses separately.

I think I might need this at some point, but for the case I mentioned I have a better idea (http://llvm-reviews.chandlerc.com/D1669)

[arsenm]

Don't have a current need for this

include/llvm/IR/DataLayout.h

public:
   /// the backends/clients are updated.
   unsigned getPointerSize(unsigned AS = 0) const {
     DenseMap<unsigned, PointerAlignElem>::const_iterator val = Pointers.find(AS);
     if (val == Pointers.end()) {
       val = Pointers.find(0);
     }
     return val->second.TypeBitWidth;
   }
+
+  /// Return the largest pointer size for known address spaces.
+  unsigned getMaxPointerSize() const {
+    unsigned MaxPtrSize = 0;
+    for (DenseMap<unsigned, PointerAlignElem>::const_iterator I
+           = Pointers.begin(), E = Pointers.end(); I != E; ++I) {
+      MaxPtrSize = std::max(MaxPtrSize, I->second.TypeBitWidth);
+    }
+
+    return (MaxPtrSize == 0) ? 64 : MaxPtrSize;
+  }
+
   /// Layout pointer size, in bits
   /// FIXME: The defaults need to be removed once all of
   /// the backends/clients are updated.
   unsigned getPointerSizeInBits(unsigned AS = 0) const {
     return getPointerSize(AS) * 8;
   }
 
   /// Layout pointer size, in bits, based on the type.  If this function is