JIT: Fix initblk/cpblk and STORE_DYN_BLK size mismatch

STORE_DYN_BLK turns into a call to JIT_MemSet/JIT_MemCpy that go quite
directly to memset/memcpy, so the size is actually a native uint. This
can cause problems since the JIT does not make any normalization
guarantees above 32 bits.

Fix #78912

src/coreclr/jit/gtlist.h

@@ -87,7 +87,7 @@ GTNODE(OBJ              , GenTreeObj         ,0,GTK_UNOP|GTK_EXOP)
 GTNODE(STORE_OBJ        , GenTreeObj         ,0,GTK_BINOP|GTK_EXOP|GTK_NOVALUE) // Object that MAY have gc pointers, and thus includes the relevant gc layout info.
 GTNODE(BLK              , GenTreeBlk         ,0,GTK_UNOP|GTK_EXOP)              // Block/object with no gc pointers, and with a known size (e.g. a struct with no gc fields)
 GTNODE(STORE_BLK        , GenTreeBlk         ,0,GTK_BINOP|GTK_EXOP|GTK_NOVALUE) // Block/object with no gc pointers, and with a known size (e.g. a struct with no gc fields)
-GTNODE(STORE_DYN_BLK    , GenTreeStoreDynBlk ,0,GTK_SPECIAL|GTK_NOVALUE)        // Dynamically sized block store
+GTNODE(STORE_DYN_BLK    , GenTreeStoreDynBlk ,0,GTK_SPECIAL|GTK_NOVALUE)        // Dynamically sized block store, with native uint size
 GTNODE(NULLCHECK        , GenTreeIndir       ,0,GTK_UNOP|GTK_NOVALUE)           // Null checks the source
 
 GTNODE(ARR_LENGTH       , GenTreeArrLen      ,0,GTK_UNOP|GTK_EXOP)            // single-dimension (SZ) array length

src/coreclr/jit/importer.cpp

@@ -10893,6 +10893,11 @@ void Compiler::impImportBlockCode(BasicBlock* block)
                         op2 = gtNewOperNode(GT_INIT_VAL, TYP_INT, op2);
                     }
 
+#ifdef TARGET_64BIT
+                    // STORE_DYN_BLK takes a native uint size as it turns into call to memset.
+                    op3 = gtNewCastNode(TYP_I_IMPL, op3, /* fromUnsigned */ true, TYP_U_IMPL);
+#endif
+
                     op1  = new (this, GT_STORE_DYN_BLK) GenTreeStoreDynBlk(op1, op2, op3);
                     size = 0;
 
@@ -10920,6 +10925,12 @@ void Compiler::impImportBlockCode(BasicBlock* block)
                 else
                 {
                     op2 = gtNewOperNode(GT_IND, TYP_STRUCT, op2);
+
+#ifdef TARGET_64BIT
+                    // STORE_DYN_BLK takes a native uint size as it turns into call to memcpy.
+                    op3 = gtNewCastNode(TYP_I_IMPL, op3, /* fromUnsigned */ true, TYP_U_IMPL);
+#endif
+
                     op1 = new (this, GT_STORE_DYN_BLK) GenTreeStoreDynBlk(op1, op2, op3);
 
                     if ((prefixFlags & PREFIX_VOLATILE) != 0)

src/coreclr/jit/morphblock.cpp

@@ -1571,9 +1571,8 @@ GenTree* Compiler::fgMorphStoreDynBlock(GenTreeStoreDynBlk* tree)
     if (tree->gtDynamicSize->IsIntegralConst())
     {
         int64_t size = tree->gtDynamicSize->AsIntConCommon()->IntegralValue();
-        assert(FitsIn<int32_t>(size));
 
-        if (size != 0)
+        if ((size != 0) && FitsIn<int32_t>(size))
         {
             GenTree* lhs = gtNewBlockVal(tree->Addr(), static_cast<unsigned>(size));
             GenTree* asg = gtNewAssignNode(lhs, tree->Data());