Hoist the nullchecks for 'this' object (#68588)

* CSE nullcheck involving 'this'

* Move the nullcheck to hoistable only

* Minor fix to display IG01 weight correctly

* Hoist nullcheck for other objects as well

Author: kunalspathak

src/coreclr/jit/codegencommon.cpp

@@ -1739,6 +1739,7 @@ void CodeGen::genGenerateMachineCode()
     {
         compiler->opts.disAsm = true;
     }
+    compiler->compCurBB = compiler->fgFirstBB;
 
     if (compiler->opts.disAsm)
     {

src/coreclr/jit/morph.cpp

@@ -5531,8 +5531,6 @@ GenTree* Compiler::fgMorphField(GenTree* tree, MorphAddrContext* mac)
             GenTree* lclVar = gtNewLclvNode(lclNum, objRefType);
             nullchk         = gtNewNullCheck(lclVar, compCurBB);
 
-            nullchk->gtFlags |= GTF_DONT_CSE; // Don't try to create a CSE for these TYP_BYTE indirections
-
             if (asg)
             {
                 // Create the "comma" node.

src/coreclr/jit/optimizer.cpp

@@ -6516,6 +6516,15 @@ void Compiler::optHoistLoopBlocks(unsigned loopNum, ArrayStack<BasicBlock*>* blo
             {
                 return false;
             }
+            else if (node->OperIs(GT_NULLCHECK))
+            {
+                // If a null-check is for `this` object, it is safe to
+                // hoist it out of the loop. Assrtionprop will get rid
+                // of left over nullchecks present inside the loop. Also,
+                // since NULLCHECK has no value, it will never be CSE,
+                // hence this check is not present in optIsCSEcandidate().
+                return true;
+            }
 
             // Tree must be a suitable CSE candidate for us to be able to hoist it.
             return m_compiler->optIsCSEcandidate(node);