Teach LiveInterval::isZeroLength about null SlotIndexes.
When instructions are deleted, they leave tombstone SlotIndex entries.
The isZeroLength method should ignore these null indexes.
This causes RABasic to sometimes spill a callee-saved register in the
abi-isel.ll test, so don't run that test with -regalloc=basic. Prioritizing
register allocation according to spill weight can cause more registers to be
used.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@131436
91177308-0d34-0410-b5e6-
96231b3b80d8