Loop canonicalization should always create loop pre-headers

[BruceForstall]

It would simplify a lot of code if loop canonicalization always creates pre-header blocks, and they are maintained late into compilation. This would require both allowing them to exist, but also to be maintained, e.g., #62665.

Another motivating issue: #68844 (comment)

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Issue Details

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It would simplify a lot of code if loop canonicalization always creates pre-header blocks, and they are maintained late into compilation. This would require both allowing them to exist, but also to be maintained, e.g., #62665.

Author:	BruceForstall
Assignees:	BruceForstall
Labels:	area-CodeGen-coreclr
Milestone:	8.0.0

[BruceForstall]

optHoistThisLoop has a bunch of special processing that could be eliminated if pre-headers always existed (and thus dominators included them).

[BruceForstall]

Should be able to remove pre-header special handling in optAddCopies, which assumes pre-headers are not in the dominator tree.

This phase was recently removed with #83310

[BruceForstall]

fgDominate also contains special handling for pre-headers that could be removed if pre-headers always exist before dominators are built.