Fix regression in runtime-jit-experimental (#70797)

The newly-introduced `emitRemoveJumpToNextInst` optimization caused
a regression when hot/cold-splitting, where jumps from the last hot
instruction to the first cold instruction were erroneously removed.
This is fixed by disabling the `isRemovableJmpCandidate` flag for
branches between hot/cold sections.

On an unrelated note, a JIT dump message has been added to indicate
stress-splitting is occurring.

src/coreclr/jit/codegenlinear.cpp

@@ -753,24 +753,27 @@ void CodeGen::genCodeForBBlist()
                 break;
 
             case BBJ_ALWAYS:
-                inst_JMP(EJ_jmp, block->bbJumpDest
+#ifdef TARGET_XARCH
+            {
+                // If a block was selected to place an alignment instruction because it ended
+                // with a jump, do not remove jumps from such blocks.
+                // Do not remove a jump between hot and cold regions.
+                bool isRemovableJmpCandidate =
+                    !block->hasAlign() && !compiler->fgInDifferentRegions(block, block->bbJumpDest);
+
 #ifdef TARGET_AMD64
-                         // AMD64 requires an instruction after a call instruction for unwinding
-                         // inside an EH region so if the last instruction generated was a call instruction
-                         // do not allow this jump to be marked for possible later removal.
-                         //
-                         // If a block was selected to place an alignment instruction because it ended
-                         // with a jump, do not remove jumps from such blocks.
-                         ,
-                         /* isRemovableJmpCandidate */ !GetEmitter()->emitIsLastInsCall() && !block->hasAlign()
+                // AMD64 requires an instruction after a call instruction for unwinding
+                // inside an EH region so if the last instruction generated was a call instruction
+                // do not allow this jump to be marked for possible later removal.
+                isRemovableJmpCandidate = isRemovableJmpCandidate && !GetEmitter()->emitIsLastInsCall();
+#endif // TARGET_AMD64
+
+                inst_JMP(EJ_jmp, block->bbJumpDest, isRemovableJmpCandidate);
+            }
 #else
-#ifdef TARGET_XARCH
-                         ,
-                         /* isRemovableJmpCandidate */ !block->hasAlign()
-#endif
+                inst_JMP(EJ_jmp, block->bbJumpDest);
+#endif // TARGET_XARCH
 
-#endif
-                             );
                 FALLTHROUGH;
 
             case BBJ_COND:

src/coreclr/jit/flowgraph.cpp

@@ -3421,6 +3421,7 @@ PhaseStatus Compiler::fgDetermineFirstColdBlock()
     {
         firstColdBlock       = fgFirstBB->bbNext;
         prevToFirstColdBlock = fgFirstBB;
+        JITDUMP("JitStressProcedureSplitting is enabled: Splitting after the first basic block\n");
     }
     else
     {