[mlir] Allow nested regions in inlineRegionAndEmitStore

[tpopp]

Summary

This is useful for scalar code that uses for/while loops.
This has also been confirmed to work for representing std.pow as an
scf.for loop on gpus.

[nicolasvasilache]

well you need at least some test here :p

mlir/lib/Dialect/Linalg/Transforms/Loops.cpp

static void inlineRegionAndEmitStore(OpType op, ArrayRef<Value> indexedValues,
                                      ArrayRef<SmallVector<Value, 8>> indexing,
                                      ArrayRef<Value> outputBuffers) {
   assert(op->getNumRegions() == 1 && "Expected single region op");
   auto &b = ScopedContext::getBuilderRef();
   auto &block = op->getRegion(0).front();
   BlockAndValueMapping map;
   map.map(block.getArguments(), indexedValues);
   for (auto &op : block.without_terminator()) {
-    assert(op.getNumRegions() == 0 && "expected a non-nested region");
     auto *newOp = b.clone(op, map);
     map.map(op.getResults(), newOp->getResults());
   }
 
   Operation &terminator = block.back();
   assert(isa<linalg::YieldOp>(terminator) &&
          "expected a yield op in the end of the region");
   for (unsigned i = 0, e = terminator.getNumOperands(); i < e; ++i) {