Instruction::isIdenticalToWhenDefined(): Check getNumOperands() in advance of std::equal(op) to appease MSVC Debug build.

MSVC Debug build is confused with (possibly invalid) op_begin(), if op_begin() == op_end().

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210000 91177308-0d34-0410-b5e6-96231b3b80d8