[CodeGenPrepare] avoid crashing from replacing a phi twice

Summary:
This is a fix to bug 41052:
https://bugs.llvm.org/show_bug.cgi?id=41052

While trying to optimize a memory instruction in a dead basic block, we end up registering the same phi for replacement twice. This patch avoids registering more than the first replacement candidate for a phi.

Patch by: JesperAntonsson

Reviewers: skatkov, aprantl

Reviewed By: aprantl

Subscribers: jdoerfert, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D59358

llvm-svn: 356260