[WebAssembly] Support running without a register allocator in the default CodeGen passes

This allows backends which don't use a traditional register allocator,
but do need PHI lowering and other passes, to use the default
TargetPassConfig::addFastRegAlloc and
TargetPassConfig::addOptimizedRegAlloc implementations.

Differential Revision: http://reviews.llvm.org/D12691

llvm-svn: 247065