[libc] Remove special case handing around test case that was fixed

Summary:
I needed to hack something around this for a previous backend bug, it's
since been fixed as far as I'm aware.

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Differential Revision: https://phabricator.intern.facebook.com/D60251346

Author: jhuber6

libc/test/src/__support/blockstore_test.cpp

@@ -183,11 +183,10 @@ TEST_F(LlvmLibcBlockStoreTest, PopulateAndIterateReverse10) {
   populate_and_iterate<4, 10, true>();
 }
 
-TEST_F(LlvmLibcBlockStoreTest, Back) { back_test<false>(); }
-
-// FIXME: Combing this test with the above test makes the AMDGPU backend
-// generate code which hangs. This should be fixed in the clang compiler.
-TEST_F(LlvmLibcBlockStoreTest, BackReverse) { back_test<true>(); }
+TEST_F(LlvmLibcBlockStoreTest, Back) {
+  back_test<false>();
+  back_test<true>();
+}
 
 TEST_F(LlvmLibcBlockStoreTest, Empty) {
   empty_test<false>();