[llvm] [MISched] Implement `UnanalyzableFrontier`-based DAG construction algorithm (PR #227375)

[Nathan Corbyn via llvm-commits]

cofibrant wrote:

Once enabled we see a 1.17% improvement in compile time geomean for `stage1-aarch64-O3`:
Benchmark | Old | New
-- | -- | --
kimwitu++        | 39086M | 39077M (-0.03%)
sqlite3          | 36449M | 36356M (-0.26%)
consumer-typeset | 34282M | 33899M (-1.12%)
Bullet           | 104543M | 103944M (-0.57%)
tramp3d-v4 | 82501M  | 82194M (-0.37%)
mafft      | 37239M  | 35515M (-4.63%)
ClamAV     | 53293M  | 53199M (-0.18%)
lencod     | 72401M  | 69608M (-3.86%)
SPASS      | 43031M  | 42811M (-0.51%)
7zip       | 202744M | 202562M (-0.09%)
geomean    | 59193M  | 58498M (-1.17%)

([Comparison](https://llvm-compile-time-tracker.com/compare.php?from=d44eb4c7fea8a287cc2021070c7b96ec443bff60&to=385c7a62eea6d24f8caf322cbbea787fefad46eb&stat=instructions:u).)

(CC @c-rhodes, @nikic). 

https://github.com/llvm/llvm-project/pull/227375