Prior load folding check on chain operand was too strict. It requires the

Prior load folding check on chain operand was too strict. It requires the
chain operand to point to the load being folded. Now we relax this, traversing
up the chain, if it doesn't reach the load, then it's ok. We will create a
TokenFactor (of all the chain operands and the load's chain) to capture all
the control flow dependencies.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@30897 91177308-0d34-0410-b5e6-96231b3b80d8