Auto merge of #109247 - saethlin:inline-without-inline, r=oli-obk

Permit MIR inlining without #[inline]

I noticed that there are at least a handful of portable-simd functions that have no `#[inline]` but compile to an assign + return.

I locally benchmarked inlining thresholds between 0 and 50 in increments of 5, and 50 seems to be the best. Interesting. That didn't include check builds though, ~maybe perf will have something to say about that~.

Perf has little useful to say about this. We generally regress all the check builds, as best as I can tell, due to a number of small codegen changes in a particular hot function in the compiler. Probably this is because we've nudged the inlining outcomes all over, and uses of `#[inline(always)]`/`#[inline(never)]` might need to be adjusted.

Author: bors