Move the code that adds the DeadMachineInstructionElimPass from

target-independent code to target-specific code. This prevents it
from running on targets that aren't using fast-isel.

In addition to saving compile time, this addresses the problem
that not all targets are prepared for it. In order to use this
pass, all instructions must declare all their fixed uses and
defs of physical registers.

llvm-svn: 58144