Remove an incorrect assert in MFMASmallGemmSingleWaveOpt.

This assert was failing in a fuzzing test. I consulted with @jrbyrnes who said:

The MFMASmallGemmSingleWaveOpt::apply() method is invoked if and only if the
user has inserted an intrinsic llvm.amdgcn.iglp.opt(i32 1) into their source
code. This intrinsic applies a highly specialized DAG mutation to result in
specific scheduling for a specific set of kernels. These assertions are really
just confirming that the characteristics of the kernel match what is expected
(i.e. The kernels are similar to the ones this DAG mutation strategy were
designed against).

However, if we apply this DAG mutation to kernels for which is was not
designed, then we may not find the types of instructions we are looking for,
and may end up with empty caches.

I think it should be fine to just return false if the cache is empty instead
of the assert.

Author: anjenner

llvm/lib/Target/AMDGPU/AMDGPUIGroupLP.cpp

@@ -1891,7 +1891,6 @@ class MFMASmallGemmSingleWaveOpt final : public IGLPStrategy {
         }
       }
 
-      assert(Cache->size());
       auto *DAG = SyncPipe[0].DAG;
       for (auto &Elt : *Cache) {
         if (DAG->IsReachable(Elt, const_cast<SUnit *>(SU)))