[MLInliner] Don't inline call sites in unreachable basic blocks

This requires DominatorTree be updated, which we do in the ml inliner
case, but not in the default case, and the cost of doing so is
noticeable to compile time for the latter[1]. So the patch only affects
the ML inliner.

[1] https://llvm-compile-time-tracker.com/compare.php?from=9fc0aa45e3312944431ba7e1ca0cec99c613992b&to=7af461b1ce0d9138211ef5f883f35d5b9ddf47be&stat=wall-time

Differential Revision: https://reviews.llvm.org/D127899