[GlobalISel] Legalizer: Retry combining illegal artifacts as long as there new artifacts

Summary:
Currently, Legalizer aborts if it’s unable to legalize artifacts. However, it’s
possible to combine them after processing the rest of the instruction because
the legalization is likely to generate more artifacts that allow ArtifactCombiner
to combine away them.

Instead, move illegal artifacts to another list called RetryList and wait until all of the
instruction in InstList are legalized. After that, check if there is any new artifacts and
try to combine them again if that’s the case. If not, abort. The idea is similar to D59339,
but the approach is a bit different.

This patch fixes the issue described above, but the legalizer still may be unable to handle
some cases depending on when to legalize artifacts. So, in the long run, we probably need
a different legalization strategy that handles this dependency in a better way.

Reviewers: dsanders, aditya_nandakumar, qcolombet, arsenm, aemerson, paquette

Reviewed By: dsanders

Subscribers: jvesely, wdng, nhaehnle, rovka, javed.absar, hiraditya, Petar.Avramovic, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D65894

llvm-svn: 369805

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