[IRGen] Add dynamic size variant of CreateMemCpy.

Similar to the existing variant which takes a fixed size, the new
variant which takes a dynamic size digs the relevant address and
alignment values out of the Address struct and passes them along.

Author: nate-chandler

lib/IRGen/IRBuilder.h

@@ -275,6 +275,13 @@ class IRBuilder : public IRBuilderBase {
         size.getValue());
   }
 
+  llvm::CallInst *CreateMemCpy(Address dest, Address src, llvm::Value *size) {
+    return CreateMemCpy(dest.getAddress(),
+                        llvm::MaybeAlign(dest.getAlignment().getValue()),
+                        src.getAddress(),
+                        llvm::MaybeAlign(src.getAlignment().getValue()), size);
+  }
+
   using IRBuilderBase::CreateMemSet;
   llvm::CallInst *CreateMemSet(Address dest, llvm::Value *value, Size size) {
     return CreateMemSet(dest.getAddress(), value, size.getValue(),