Change constant folding APIs to take an optional TargetData, and change

Change constant folding APIs to take an optional TargetData, and change
ConstantFoldInstOperands/ConstantFoldCall to take a pointer to an array
of operands + size, instead of an std::vector.

In some cases, switch to using a SmallVector instead of a vector.
This allows us to get rid of some special case gross code that was there
to avoid the cost of constructing a vector.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@33670 91177308-0d34-0410-b5e6-96231b3b80d8