fix a valgrind error on copy-constructor-synthesis.cpp, which is caused when

fix a valgrind error on copy-constructor-synthesis.cpp, which is caused when
the custom insertion hook deletes the instruction, then we try to set dead
flags on it.  Neither the code that I added nor the code that was there
before was safe.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@99538 91177308-0d34-0410-b5e6-96231b3b80d8