This is to fix the bug in PR40061 related with https://reviews.llvm.org/rL339035.
In https://reviews.llvm.org/rL339035, live interval of source pseudo register in rematerialized copy is saved in ToBeUpdated set and its update is postponed.
In PR40061, %t2 = %t1 is rematerialized and %t1 is added into toBeUpdated set to postpone its live interval update. After the rematerialization, the live interval of %t1 is larger than necessary. Then %t1 is merged into %t3 and %t1 gets removed. After the merge, %t3 contains live interval larger than necessary. Because %t3 is not in toBeUpdated set, its live interval is not updated after register coalescing and it will break some assumption in regalloc.
The patch requires the live interval of destination register in a merge to be updated if the source register is in ToBeUpdated.