[CodeGen] Rename MachineInstrBuilder::addOperand. NFC

Rename from addOperand to just add, to match the other method that has been
added to MachineInstrBuilder for adding more than just 1 operand.

See https://reviews.llvm.org/D28057 for the whole discussion.

Differential Revision: https://reviews.llvm.org/D28556

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@291891 91177308-0d34-0410-b5e6-96231b3b80d8
66 files changed