Make RAFast::UsedInInstr indexed by register units.

This fixes some problems with too conservative checking where we were
marking all aliases of a register as used, and then also checking all
aliases when allocating a register.

<rdar://problem/13249625>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@175782 91177308-0d34-0410-b5e6-96231b3b80d8