Auto merge of #115194 - tmiasko:inline-always-encode-mir, r=compiler-errors

Fix inlining with -Zalways-encode-mir

Only inline functions that are considered eligible for inlining
by the reachability pass.

This constraint was previously indirectly enforced by only exporting MIR
of eligible functions, but that approach doesn't work with
-Zalways-encode-mir enabled.

Author: bors