Jit: fix some sources of x64 chk/rel diffs (dotnet#39888)

Break IGs for perfscores the same way in chk and rel. Also, fix the xarch
peephole to work across extended IGs.

Closes dotnet#39845.

src/coreclr/src/jit/codegenlinear.cpp

@@ -344,7 +344,6 @@ void CodeGen::genCodeForBBlist()
  needLabel = true;
  }
 
-#if defined(DEBUG) || defined(LATE_DISASM)
  // We also want to start a new Instruction group by calling emitAddLabel below,
  // when we need accurate bbWeights for this block in the emitter. We force this
  // whenever our previous block was a BBJ_COND and it has a different weight than us.
@@ -356,7 +355,6 @@ void CodeGen::genCodeForBBlist()
  {
  needLabel = true;
  }
-#endif // DEBUG || LATE_DISASM
 
  if (needLabel)
  {

src/coreclr/src/jit/emitxarch.cpp

@@ -163,8 +163,10 @@ bool emitter::IsDstSrcSrcAVXInstruction(instruction ins)
 
 bool emitter::AreUpper32BitsZero(regNumber reg)
 {
- // Don't look back across IG boundaries (possible control flow)
- if (emitCurIGinsCnt == 0)
+ // If there are no instructions in this IG, we can look back at
+ // the previous IG's instructions if this IG is an extension.
+ //
+ if ((emitCurIGinsCnt == 0) && ((emitCurIG->igFlags & IGF_EXTEND) == 0))
  {
  return false;
  }