[SmallVector] Optimize move assignment operator for N==0 case

Due to the SmallVector hierarchy, N==0 cannot be leveraged by functions defined
in base classes. This patch special cases N==0 for SmallVector to save code size
and be slightly more efficient.

In a Release build of x86 only clang, .text is -3.34KiB smaller. In lld .text is
7.17KiB smaller.

Reviewed By: lichray

Differential Revision: https://reviews.llvm.org/D117976