[AMDGPU] Fix a crash when access beyond the end of the basic block

[qishilu]

Summary

the phi instruction is at front of the basic block 。
and the loop will out of bound if the use instruction is a phi instruction

the crash log at 50617
(I don't have commit access)

[foad]

Needs a test case.

llvm/lib/Target/AMDGPU/SIInstrInfo.cpp

7736

It's not OK to just remove this assert. After the first loop (line 7717) NumUses should be set to the number of uses of VReg in DefBB. The second loop should return false as soon as it has seen all the uses, so it should not fall off the end of the BB.

llvm/lib/Target/AMDGPU/SIInstrInfo.cpp

 
   // Stop scan when we have seen all the uses.
   for (auto I = std::next(DefMI.getIterator()); ; ++I) {
-    assert(I != DefBB->end());
+    if(I == DefBB->end())
+      return true;
 
     if (I->isDebugInstr())
       continue;