Initially, slot indexes are quad-spaced. There is room for inserting up to 3
new instructions between the original instructions.
When we run out of indexes between two instructions, renumber locally using
double-spaced indexes. The original quad-spacing means that we catch up quickly,
and we only have to renumber a handful of instructions to get a monotonic
sequence. This is much faster than renumbering the whole function as we did
before.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@127023 91177308-0d34-0410-b5e6-96231b3b80d8
