[Hexagon] Fix VTCM allocation (#8954)

Check if a buffer is in the `vtcm_buffers` list, before it's removed
from it.

Author: Krzysztof Parzyszek

python/tvm/contrib/hexagon.py

@@ -194,9 +194,10 @@ def mutate(stmt):
         if isinstance(stmt, tvm.tir.Allocate):
             var = stmt.buffer_var
             scope = var.type_annotation.storage_scope
+            is_vtcm = var in vtcm_buffers
             if scope == "local.vtcm":
                 vtcm_buffers.pop()
-            if var in vtcm_buffers:
+            if is_vtcm:
                 is_null = tvm.tir.call_intrin("bool", tvm.ir.Op.get("tir.isnullptr"), var)
                 throw_error = tvm.tir.call_intrin(
                     "int32", tvm.ir.Op.get("tir.tvm_throw_last_error")