misched: Target-independent support for MacroFusion.

Uses the infrastructure from r167742 to support clustering instructure
that the target processor can "fuse". e.g. cmp+jmp.

Next step: target hook implementations with test cases, and enable.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@167744 91177308-0d34-0410-b5e6-96231b3b80d8