[mips] Make NOP a pseudo instruction and expand it to "sll $zero, $zero, 0".


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174546 91177308-0d34-0410-b5e6-96231b3b80d8
Remove some dead code.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174547 91177308-0d34-0410-b5e6-96231b3b80d8
Clean up multiple skeleton compile units if we have multiple compile
units coming in.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174548 91177308-0d34-0410-b5e6-96231b3b80d8