Fix coalescing with IMPLICIT_DEF values.

PHIElimination inserts IMPLICIT_DEF instructions to guarantee that all
PHI predecessors have a live-out value. These IMPLICIT_DEF values are
not considered to be real interference when coalescing virtual
registers:

  %vreg1 = IMPLICIT_DEF
  %vreg2 = MOV32r0

When joining %vreg1 and %vreg2, the IMPLICIT_DEF instruction and its
value number should simply be erased since the %vreg2 value number now
provides a live-out value for the PHI predecesor block.

llvm-svn: 165813