Add a new SSA-based peephole optimizer which includes copy propagation and

Add a new SSA-based peephole optimizer which includes copy propagation and
folding of instructions into addressing modes.  This creates lots of dead
instructions, which are currently not deleted.  It also creates a lot of
instructions that the X86 backend currently cannot handle.  :(

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@10275 91177308-0d34-0410-b5e6-96231b3b80d8