[PATCH] Allow blocks to be merged when one has an undef input to a phi.

[Mark Lacey]

Can someone please review, and commit if everything looks good?

Thanks,

Mark

Description:
    Allow blocks to be merged when one has an undef input to a phi.
    
    TryToSimplifyUncondBranchFromEmptyBlock was checking that any common
    predecessors of the two blocks it is attempting to merge supply the
    same incoming values to any phi in the successor block. This change
    allows merging in the case where there is one or more incoming values
    that are undef. The undef values are rewritten to match the non-undef
    value that flows from the other edge.
    
    <rdar://problem/12801861>

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