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

and assert when mask is too large to apply in the small case,
previously the extra words were silently ignored.
clang-format the entire function to match current code standards.

This is a rewrite of r247972 which was reverted in r247983 due to
warning and possible UB on 32-bits hosts.

llvm-svn: 247993