[NaryReassociate] Detect deleted instr with WeakVH

Summary:
If NaryReassociate succeed it will, when replacing the old instruction
with the new instruction, also recursively delete trivially
dead instructions from the old instruction. However, if the input to the
NaryReassociate pass contain dead code it is not save to recursively
delete trivially deadinstructions as it might lead to deleting the newly
created instruction.

This patch will fix the problem by using WeakVH to detect this
rare case, when the newly created instruction is dead, and it will then
restart the basic block iteration from the beginning.

This fixes pr37539

Reviewers: tra, meheff, grosser, sanjoy

Reviewed By: sanjoy

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D47139

llvm-svn: 333155