Quick: Fix DCE to rename SSA regs for degenerate Phis.

If we're inserting a new Phi and all its inputs are the same
SSA reg (for example, we allow a single-input Phi), some
subsequent insn may actually refer to that reg, so we need
to rename it to keep the graph in a consistent state.

Change-Id: Ic6a1907c3138f4a7d3b13f9e58e9107ca2d92f17