Add VirtRegMap::rewrite() and use it in the new register allocators.

Add VirtRegMap::rewrite() and use it in the new register allocators.

The rewriter works almost identically to -rewriter=trivial, except it also
eliminates any identity copies.

This makes the new register allocators independent of VirtRegRewriter.cpp which
will be going away at the same time as RegAllocLinearScan.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@125967 91177308-0d34-0410-b5e6-96231b3b80d8