Replacing is with == for the dtype check.

>>> a = np.ndarray([1,1]).astype(np.half)
>>> a
array([[0.007812]], dtype=float16)
>>> a.dtype
dtype('float16')
>>> a.dtype == np.half
True
>>> a.dtype == np.float16
True
>>> a.dtype is np.float16
False

Checking with `is` leads to inconsistency in checking.

Reviewed By: silvas

Differential Revision: https://reviews.llvm.org/D139121

Author: Prashant Kumar

mlir/python/mlir/runtime/np_to_memref.py

@@ -23,13 +23,14 @@ class F16(ctypes.Structure):
   _fields_ = [("f16", ctypes.c_int16)]
 
 
+# https://stackoverflow.com/questions/26921836/correct-way-to-test-for-numpy-dtype
 def as_ctype(dtp):
   """Converts dtype to ctype."""
-  if dtp is np.dtype(np.complex128):
+  if dtp == np.dtype(np.complex128):
     return C128
-  if dtp is np.dtype(np.complex64):
+  if dtp == np.dtype(np.complex64):
     return C64
-  if dtp is np.dtype(np.float16):
+  if dtp == np.dtype(np.float16):
     return F16
   return np.ctypeslib.as_ctypes_type(dtp)