[MLIR] Work around an ICE in GCC 7.

Looks like we have a blind spot in the testing matrix.

AsyncRegionRewriter.cpp: In member function ‘virtual void {anonymous}::GpuAsyncRegionPass::runOnFunction()’:
AsyncRegionRewriter.cpp:113:16: internal compiler error: in replace_placeholders_r, at cp/tree.c:2804
   if (getFunction()
       ~~~~~~~~~~~~~
           .getRegion()
           ~~~~~~~~~~~~
           .walk(Callback{OpBuilder{&getContext()}})
           ~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Author: d0k

mlir/lib/Dialect/GPU/Transforms/AsyncRegionRewriter.cpp

@@ -108,10 +108,8 @@ struct GpuAsyncRegionPass::Callback {
 // inserts the necessary synchronization (as gpu.wait ops). Assumes sequential
 // execution semantics and that no GPU ops are asynchronous yet.
 void GpuAsyncRegionPass::runOnFunction() {
-  if (getFunction()
-          .getRegion()
-          .walk(Callback{OpBuilder(&getContext())})
-          .wasInterrupted())
+  Callback callback{OpBuilder(&getContext())};
+  if (getFunction().getRegion().walk(callback).wasInterrupted())
     return signalPassFailure();
 }