[AMDGPU] fix test to survive the most basic undef constant folding

This will likely need to be changed again for anything more than:
fmul undef, undef -> undef

llvm-svn: 327034
diff --git a/llvm/test/CodeGen/AMDGPU/debug-value.ll b/llvm/test/CodeGen/AMDGPU/debug-value.ll
index 30c1342..8fa05f6 100644
--- a/llvm/test/CodeGen/AMDGPU/debug-value.ll
+++ b/llvm/test/CodeGen/AMDGPU/debug-value.ll
@@ -30,7 +30,7 @@
 bb21:                                             ; preds = %bb
   %tmp22 = fmul <4 x float> %tmp18, undef
   %tmp23 = fadd <4 x float> undef, %tmp22
-  %tmp24 = fmul <4 x float> undef, undef
+  %tmp24 = fmul <4 x float> %tmp23, undef
   br label %bb28
 
 bb25:                                             ; preds = %bb