[CGP] Fix the remove of matched phis in complex addressing mode

When we replace the Phi we created with matched ones it is possible that
there are two identical phi nodes in IR. And matcher is smart enough to find that
new created phi matches both of them. So we try to replace our phi node with
matched ones twice and what is bad we delete our phi node twice causing a crash.

As soon as we found that we have two identical Phi nodes it makes sense to do
a clean-up and replace one phi node by other one.
The patch implements it.

Reviewers: john.brawn, reames
Reviewed By: john.brawn
Subscribers: llvm-commits
Differential Revision: https://reviews.llvm.org/D43758

llvm-svn: 327250