[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()}})
           ~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~