Relax assertion

Previously mergeUseFrom() requires both arguments to be used and their
LVInfo have been constructed when mergeUseFrom() is invoked. This isn't
true for inline asm call, in which the second argument is dead in term
of llvm value usage (but call isn't dead). Relax it to let this case
compile without assertion.

Author: jgu222

IGC/Compiler/CISACodeGen/LiveVars.cpp

@@ -714,8 +714,14 @@ bool LiveVars::hasInterference(llvm::Value* V0, llvm::Value* V1)
 // updated to reflect such a merging.
 void LiveVars::mergeUseFrom(Value* V, Value* fromV)
 {
-    IGC_ASSERT_MESSAGE(VirtRegInfo.count(V), "MergeUseFrom should be used after LVInfo has been contructed!");
-    IGC_ASSERT_MESSAGE(VirtRegInfo.count(fromV), "MergeUseFrom should be used after LVInfo has been contructed!");
+    // Normally, both VitrRegInfo.count(V) and VirtRegInfo.count(fromV) are
+    // non-zero, meaning their LVInfo have been constructed (they are not
+    // dead). However, we see inline asm case in which fromV is dead in term
+    // of llvm value usage and therefore no LVInfo (as inline asm call has
+    // side-effect, the inline asm is not dead). For this reason, we relax
+    // assertion to require at least one has LVInfo.
+    IGC_ASSERT_MESSAGE(VirtRegInfo.count(V) || VirtRegInfo.count(fromV),
+        "MergeUseFrom should be used after LVInfo have been constructed!");
 
     LVInfo& LVI = getLVInfo(V);
     LVInfo& fromLVI = getLVInfo(fromV);