removePartialRedundancy was using extendToIndices on each subrange
without passing in an Undefs vector containing main reg defs that are
undef in the subrange, causing the above assert.
Unfortunately I can only reproduce this in an ll test. Turning it into a
mir test makes the problem go away.
Change-Id: I4f3ba2afb20d79f9467afb3d882f0328d38531be
I tried running the test case on trunk now and it doesn't trigger the verifier even without the fix in this patch so something must have changed so the "wanted" situation doesn't happen with this ll-input anymore.