[PATCH] Early Target Dependent DAG Combine

[Quentin Colombet]

Hi,

Currently in DAG combiner, we have roughly the following workflow for each Selection DAG node:
1. Target-independent DAG combine
2. Target-specific DAG combine

I'd like to add a target-specific DAG combine pass before any target-independent, i.e.,
1. Target-specific DAG combine
2. Target-independent DAG combine
3. Target-specific DAG combine

Attached is a patch that realizes that.

The patch uses the exact same construction as the existing "late" target-specific DAG combine hook. In other words, it defines three new methods in the base class of target lowering:
- One to set which types of SDNode should be processed through the "early" target specific DAG combine
- One to query whether a type of SDNode has been marked as should be "early" DAG combined.
- One virtual method that is the hook to the "early" DAG combine processing.
And adds the call to the related hook in DAGCombiner.

Thanks for your reviews.

-Quentin
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