Miscellaneous Fixes for SparseBitVector

Summary:

1. Fix return value in `SparseBitVector::operator&=`.
2. Add checks if SBV is being assigned is invoking SBV.

Reviewers: dberlin

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D11342

Committed on behalf of sl@

llvm-svn: 242693