Simplify SmallBitVector::applyMask by consolidating common code for 32-bit and 64-bit builds.

Extend mask value to 64 bits before taking its complement and assert when mask is
too large to apply in the small case (previously the extra words were silently ignored).

http://reviews.llvm.org/D11890

Patch by James Touton!

llvm-svn: 247972