Fix handling of the regparm attribute in the presence of classes with copy

constructors.

When I first moved regparm support to TargetInfo.cpp I tried to isolate it
in classifyArgumentTypeWithReg, but it is actually a lot easier to flip the
code around and check for regparm at the end of the decision tree.

Without this refactoring classifyArgumentTypeWithReg would have to duplicate
the logic about when to use non-byval indirect arguments.

llvm-svn: 166266