Allow targets to inject passes before the virtual register rewriter.

Allow targets to inject passes before the virtual register rewriter.

Such passes can be used to tweak the register assignments in a
target-dependent way, for example to avoid write-after-write
dependencies.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@159209 91177308-0d34-0410-b5e6-96231b3b80d8