[mips] Use "or $r0, $r1, $zero" instead of "addu $r0, $zero, $r1" to copy

physical register $r1 to $r0.

GNU disassembler recognizes an "or" instruction as a "move", and this change
makes the disassembled code easier to read.

Original patch by Reed Kotler.

llvm-svn: 170655