[PATCH] Enforce that the SDNode inside SDUse is not null

[Pete Cooper]

Hi Hal

This is a spin off from http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20150622/284125.html <http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20150622/284125.html>.

Although an SDValue can contain a null SDNode*, it is useful for defining iterators to know that SDNode’s operands cannot be null SDNode’s.  Specifically, this patch ensures that the SDUse’s used as operands are never set to point to a null SDNode*.

The only situation where this is allowed is immediately prior to deleting a node when we need to clear the node operands.  For this i introduced a reset method on SDUse with a note to say when it should be used.

Cheers
Pete

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