Make the assignment operator for SmallPtrSet return a reference, and fix a long-standing bug in the copy
ctor while I'm at it.

Thanks to Chris Lattner for help with this patch.

llvm-svn: 38470

Patch by Chis Lattner.

llvm-svn: 38467

llvm-svn: 38460

llvm-svn: 37697

 1. Make SmallPtrSet::erase faster in the small case by replacing a memmove
    with a pointer copy.
 2. Fix a bug where the null terminator at the end of the array in the small
    case was not copied

llvm-svn: 37696

llvm-svn: 36016

llvm-svn: 33973

element was in the set.

llvm-svn: 33931

llvm-svn: 33581

llvm-svn: 33578

llvm-svn: 33577