Reapply r332907 "[GlobalISel] Improving InstructionSelect's performance by reducing MatchTable..."

Apparently the compile time problem was caused by the fact that not
all compilers / STL implementations can automatically convert
std::unique_ptr<Derived> to std::unique_ptr<Base>. Fixed (hopefully)
by making sure it's std::unique_ptr<Derived>&& (rvalue ref) to
std::unique_ptr<Base> conversion instead.

llvm-svn: 332917