[CodeGenPrepare] avoid crashing from replacing a phi twice
This is a fix to bug 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
Differential Revision: https://reviews.llvm.org/D59358