llvm/ObjCARC: Eliminate inlined AutoreleaseRV calls

Pair up inlined AutoreleaseRV calls with their matching RetainRV or
ClaimRV.

- RetainRV cancels out AutoreleaseRV.  Delete both instructions.
- ClaimRV is a peephole for RetainRV+Release.  Delete AutoreleaseRV and
  replace ClaimRV with Release.

This avoids problems where more aggressive inlining triggers memory
regressions.

This patch is happy to skip over non-callable instructions and non-ARC
intrinsics looking for the pair.  It is likely sound to also skip over
opaque function calls, but that's harder to reason about, and it's not
relevant to the goal here: if there's an opaque function call splitting
up a pair, it's very unlikely that a handshake would have happened
dynamically without inlining.

Note that this patch also subsumes the previous logic that looked
backwards from ReleaseRV.

https://reviews.llvm.org/D70370
rdar://problem/46509586