When coloring registers for a branch instruction, also make sure that
the values passed as EBB arguments are in the registers expected by the
EBB.
The first time a branch to an EBB is processed, assign the EBB arguments
to the registers where the branch arguments already reside so no
regmoves are needed.
Ghost instructions don't generate code, but they can keep registers
alive. The coloring pass needs to process values killed by ghost
instructions so it knows when the registers are freed up.
Also track register pressure changes from ghost kills in the spiller.
The EntityRef trait is used by more than just the EntityMap now, so it
should live in its own module.
Also move the entity_impl! macro into the new module so it can be used
for defining new entity references anywhere.
