Revert r187191, which broke opt -mem2reg on the testcases included in PR16867.

However, opt -O2 doesn't run mem2reg directly so nobody noticed until r188146
when SROA started sending more things directly down the PromoteMemToReg path.

In order to revert r187191, I also revert dependent revisions r187296, r187322
and r188146. Fixes PR16867. Does not add the testcases from that PR, but both
of them should get added for both mem2reg and sroa when this revert gets
unreverted.

llvm-svn: 188327