[llvm] Metadata: Optimize metadata queries (PR #70700)

[Nikita Popov via llvm-commits]

nikic wrote:

> In my ad-hoc benchmarking I am measuring `clang -O3 -c llvm-test-suite/CTMark/sqlite3/sqlite3.c` compile-time as instructions counted by valgrind/callgrind:
> 
> ```
> Before e3cf80c              37,821,687,947
> After 3cf80c                38,133,312,923
> this change before e3cf80c  37,268,867,198
> this change after 3cf80c    37,268,867,064
> ```

I just tested this as well, and the numbers I'm seeing aren't quite this good. On sqlite3 in O3 configuration, the original change was a +0.66% regression, and this one is a -0.33% win. (The geomean was +0.68% and -0.20%).

https://github.com/llvm/llvm-project/pull/70700