Add preservation of SlotIndexes to PHIElimination.
Avoid recomputing an inserted instruction's SlotIndex.
Remove verification after PHIElimination when using LiveIntervals, and move it
after the two-address pass. The remaining problems in 'make check' are occurring
later.
Fix a misunderstanding about how RegMaskBlocks works. This was caught by
assertions in the register allocator when running 'make check' without
LiveVariables.
Make SlotIndex::getEntry() return unsigned to match IndexListEntry.
Add SlotIndexes::repairIndexesInRange(), which repairs SlotIndexes after adding
and removing instructions. The implementation seems more complicated than it
needs to be, but I couldn't find something simpler that dealt with all of the
corner cases.

Also add a call to repairIndexesInRange() from repairIntervalsInRange().
Modify LiveInterval::addRange() to match the comment about what it returns.
Move the computation of the IsEarlyClobber flag into its own loop, since the
correct value is needed in every iteration of the loop for updating
LiveIntervals.
Add support to the two-address pass for updating LiveIntervals in many of the
common transformations. This includes updating repairIntervalsInRange() to
handle more cases.