fix a long standing wart: all the ComplexPattern's were being

fix a long standing wart: all the ComplexPattern's were being
passed the root of the match, even though only a few patterns
actually needed this (one in X86, several in ARM [which should
be refactored anyway], and some in CellSPU that I don't feel
like detangling).   Instead of requiring all ComplexPatterns to
take the dead root, have targets opt into getting the root by
putting SDNPWantRoot on the ComplexPattern.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@114471 91177308-0d34-0410-b5e6-96231b3b80d8