JIT: hoist loop invariant null checks (and other exception causing trees) if they happen before other side effects

[AndyAyersMS]

In https://github.com/dotnet/runtime/issues/74081#issuecomment-1555407459 we see a loop where the very first thing in the loop body is a loop-invariant null check. We should be able to hoist this and other similar exception-causing trees into the preheader provided the exceptions happen before any side effect.

[ghost]

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Issue Details

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In https://github.com/dotnet/runtime/issues/74081#issuecomment-1555407459 we see a loop where the very first thing in the loop body is a loop-invariant null check. We should be able to hoist this and other similar exception-causing trees into the preheader provided the exceptions happen before any side effect.

Author:	AndyAyersMS
Assignees:	-
Labels:	`area-CodeGen-coreclr`, `untriaged`
Milestone:	-

[AndyAyersMS]

@BruceForstall FYI cc @dotnet/jit-contrib

[jakobbotsch]

In #74081 (comment) we see a loop where the very first thing in the loop body is a loop-invariant null check.

68588 was supposed to add support for this, we should check why it doesn't kick in.