Track new virtual registers by register number.

Track new virtual registers by register number.

Track new virtual registers by register number, rather than by the live
interval created for them. This is the first step in separating the
creation of new virtual registers and new live intervals.  Eventually
live intervals will be created and populated on demand after the virtual
registers have been created and used in instructions.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188434 91177308-0d34-0410-b5e6-96231b3b80d8