[PATCH] GVN: Don't crash on an unreachable predecessor

[Duncan P. N. Exon Smith]

Don't crash when a PHI node comes from an unreachable predecessor.
Fixes (?) PR21625 [1].

I'm not really a memdep expert, so I'm not sure whether this counts
as a fix or if this just papers over a problem that will show up
some other way.

In the attached testcase, bb1 hits `PredTranslationFailure` twice,
causing the old assertion to fire.  My proposed fix is to weaken
the assertion when the basic block has no predecessors:

-      assert(I->getResult().isNonLocal() &&
+      assert((I->getResult().isNonLocal() ||
+              (I->getResult().isUnknown() && pred_begin(BB) == pred_end(BB))) &&

This approximates the logic for the entry block, which skips this
logic entirely with an early return:

    // If this is the magic first block, return this as a clobber of the whole
    // incoming value.  Since we can't phi translate to one of the predecessors,
    // we have to bail out.
    if (SkipFirstBlock)
      return true;

Thoughts?

[1]: http://llvm.org/bugs/show_bug.cgi?id=21625

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