[AVR] Support 16-bit LDS/STS on AVRTiny.

LDS/STS are 32-bit instructions on AVR, which can access up to
64KB data space. While they are 16-bit instructions on AVRTiny,
which can only access 128B data space.

Reviewed By: aykevl

Differential Revision: https://reviews.llvm.org/D139621