Fix verification of explicit operands.

Fix verification of explicit operands.

The machine code verifier did not check for explicit operands correctly. It
used MachineInstr::getNumExplicitOperands, but that method may cheat and use
the declared count in the TargetInstrDesc.

Now we check the explicit operands one at a time in visitMachineOperand.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@82652 91177308-0d34-0410-b5e6-96231b3b80d8