Teach tblgen to build permutations of instructions, so that the target author

Teach tblgen to build permutations of instructions, so that the target author
doesn't have to specify them manually.  It currently handles associativity,
e.g. knowing that (X*Y)+Z  also matches  X+(Y*Z)  and will be extended in
the future.

It is smart enough to not introduce duplicate patterns or patterns that can
never match.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@23526 91177308-0d34-0410-b5e6-96231b3b80d8