[mips] Use ADDu instead of OR to copy general purpose registers. Also, delete

[mips] Use ADDu instead of OR to copy general purpose registers. Also, delete
the InstAlias pattern which maps "move" to OR to resolve ambiguity in
MatchTable.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@186855 91177308-0d34-0410-b5e6-96231b3b80d8