LLVM Value/Use does and MachineRegisterInfo/MachineOperand does.
This allows constant time for all uses list maintenance operations.

The idea was suggested by Chris. Reviewed by Evan and Dan.
Patch is tested and approved by Dan.

On normal use-cases compilation speed is not affected. On very big basic
blocks there are compilation speedups in the range of 15-20% or even better. 

llvm-svn: 48822