Auto merge of #39648 - Aatch:mir-inlining-2, r=eddyb

[MIR] Implement Inlining

Fairly basic implementation of inlining for MIR. Uses conservative
heuristics for inlining.

Doesn't handle a number of cases, but can be extended later. This is basically the same as the previous inlining PR, but without the span-related changes (as the bugs it was dealing with have since been fixed).

/cc @rust-lang/compiler

diff --cc src/librustc_driver/driver.rs
Simple merge