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.

(cherry picked from commit f60715c2)

Change-Id: I46822b0db21c187601b79d2ff1a650494558772a