Revert "[SCEV] Look through single value PHIs."

This reverts commit e441b7a7.

This patch causes a compile error in tensorflow opensource project. The stack trace looks like:

Point of crash:
llvm/include/llvm/Analysis/LoopInfoImpl.h : line 35

(gdb) ptype *this
type = const class llvm::LoopBase<llvm::BasicBlock, llvm::Loop> [with BlockT = llvm::BasicBlock, LoopT = llvm::Loop]

(gdb) p *this
$1 = {ParentLoop = 0x0, SubLoops = std::vector of length 0, capacity 0, Blocks = std::vector of length 0, capacity 1,
  DenseBlockSet = {<llvm::SmallPtrSetImpl<llvm::BasicBlock const*>> = {<llvm::SmallPtrSetImplBase> = {<llvm::DebugEpochBase> = {Epoch = 3}, SmallArray = 0x1b2bf6c8, CurArray = 0x1b2bf6c8,
        CurArraySize = 8, NumNonEmpty = 0, NumTombstones = 0}, <No data fields>}, SmallStorage = {0xfffffffffffffffe, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0}}, IsInvalid = true}

(gdb) p *this->DenseBlockSet->CurArray
$2 = (const void *) 0xfffffffffffffffe

I will try to get a case from tensorflow or use creduce to get a small case.