[llvm] [tools][llc] Add `--save-stats` option (PR #163967)

[Tomer Shafir via llvm-commits]

tomershafir wrote:

> Just to mention another possible alternative, @tobias-stadler has been working on emitting statistics for each function as a remark, which would allow for higher granularity.

I think this can be viewed as an implementation detail here, which would still require LLVM statistics semantics as described here on top, so I dont think it should block this enhancement.

https://github.com/llvm/llvm-project/pull/163967