[NewGVN] Optimize processing for instructions found trivially dead.

Don't call `isTriviallyDeadInstructions()` once we discover that
an instruction is dead. Instead, set DFS number zero (as suggested
by Danny) and forget about it (this also speeds up things as we
won't try to reprocess that block).

Differential Revision:  https://reviews.llvm.org/D28930

llvm-svn: 292676