Auto merge of #29943 - brson:inline-threshold, r=nrc

Corresponds directly to llvm's inline-threshold.

I want this so I can experiment out-of-tree with tweaking optimization settings, and this is the most important value that isn't exposed. I can't get it to work either via `-C llvm-args`.

cc @rust-lang/compiler

diff --cc src/librustc_trans/back/write.rs
Simple merge