WebAssembly: mark a few new failures

A recent change introduced this assertion failure in some corner cases.

Repro:
mkdir /s/wasm/torture-out ; time /s/wasm/waterfall/src/compile_torture_tests.py --c /s/llvm/out/bin/clang --cxx /s/llvm/out/bin/clang++ --testsuite /s/gcc/gcc/testsuite --fails /s/llvm/llvm/lib/Target/WebAssembly/known_gcc_test_failures.txt --out /s/wasm/torture-out

Or look on the wasm integration bot:
https://build.chromium.org/p/client.wasm.llvm/console

llvm-svn: 257733

llvm/lib/Target/WebAssembly/known_gcc_test_failures.txt

@@ -1,5 +1,13 @@
 # Tests which are known to fail from the GCC torture test suite.
 
+# WebAssemblyCFGStackify.cpp:438: void PlaceMarkers(llvm::MachineFunction&, const llvm::MachineLoopInfo&, const llvm::WebAssemblyInstrInfo&, llvm::MachineDominatorTree&): Assertion `ScopeTops[Stack.back()->getNumber()] == &MBB && "Block should be balanced"' failed.
+ashldi-1.c
+ashrdi-1.c
+builtin-bitops-1.c
+loop-11.c
+lshrdi-1.c
+mode-dependent-address.c
+
 # Core dump.
 920908-1.c
 pr38151.c