[mips][ias] Moved most instruction emission helpers to MipsTargetStreamer. NFC.
Summary:
* Moved all the emit*() helpers to MipsTargetStreamer.
* Moved createNop() to MipsTargetStreamer as emitNop() and emitEmptyDelaySlot().
This instruction has been split to distinguish between the 'nop' instruction
and the nop used in delay slots which is sometimes a different nop to the
'nop' instruction (e.g. for short delay slots on microMIPS).
* Moved createAddu() to MipsTargetStreamer as emitAddu().
* Moved createAppropriateDSLL() to MipsTargetStreamer as emitDSLL().
Reviewers: sdardis
Subscribers: dsanders, sdardis, llvm-commits
Differential Revision: http://reviews.llvm.org/D19712
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@268041
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