Ignore NumberHack and give each SubRegIndex instance a unique enum value instead.

Ignore NumberHack and give each SubRegIndex instance a unique enum value instead.

This passes lit tests, but I'll give it a go through the buildbots to smoke out
any remaining places that depend on the old SubRegIndex numbering.

Then I'll remove NumberHack entirely.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@104615 91177308-0d34-0410-b5e6-96231b3b80d8