Fix another crasher in SROA, reported by Joel.

We require that the indices into the use lists are stable in order to
build fast lookup tables to locate a particular partition use from an
operand of a PHI or select. This is (obviously in hind sight)
incompatible with erasing elements from the array. Really, we don't want
to erase anyways. It is expensive, and a rare operation. Instead, simply
weaken the contract of the PartitionUse structure to allow null Use
pointers to represent dead uses. Now we can clear out the pointer to
mark things as dead, and all it requires is adding some 'continue'
checks to the various loops.

I'm still reducing a test case for this, as the test case I have is
huge. I think this one I can get a nice test case for though, as it was
much more deterministic.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@165032 91177308-0d34-0410-b5e6-96231b3b80d8

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