[ARM] CodeGen: Remove AddDefaultPred. NFC.

Replace all uses of AddDefaultPred with MachineInstrBuilder::add(predOps()).
This makes the code building MachineInstrs more readable, because it allows us
to write code like:

MIB.addSomeOperand(blah)
   .add(predOps())
   .addAnotherOperand(blahblah)

instead of

AddDefaultPred(MIB.addSomeOperand(blah))
    .addAnotherOperand(blahblah)

This commit also adds the predOps helper in the ARM backend, as well as the add
method taking a variable number of operands to the MachineInstrBuilder.

The transformation has been done mostly automatically with a custom tool based
on Clang AST Matchers + RefactoringTool.

Differential Revision: https://reviews.llvm.org/D28555

llvm-svn: 291890