[mips] Simplify code in function Filler::findDelayInstr.

1. Define and use function terminateSearch.
2. Use MachineBasicBlock::iterator instead of MachineBasicBlock::instr_iterator.
3. Delete the line which checks whether an instruction is a pseudo.
The operand listing is very much outdated.
The ARM NEON vector compare instructions take three arguments. However, the
assembler should also accept a two arg form, as the docuemntation specifies that
the first (destination) register is optional.

This patch uses TwoOperandAliasConstraint to add the two argument form.

It also fixes an 80-column formatting problem in:
  test/MC/ARM/neon-bitwise-encoding

<rdar://problem/12909419> Clang rejects ARM NEON assembly instructions
[mips] Fix comments and coding style violations. Declare functions to be const.