tsan: optimize __tsan_read/write16

These callbacks are used for SSE vector accesses.
In some computational programs these accesses dominate.
Currently we do 2 uninlined 8-byte accesses to handle them.
Inline and optimize them similarly to unaligned accesses.
This reduces the vector access benchmark time from 8 to 3 seconds.

Depends on D112603.

Reviewed By: melver

Differential Revision: https://reviews.llvm.org/D114594