hdl.ast: simplify {bit,word}_select with constant offset.

We don't have any other convenient shortcut for x[off*w:(off+1)*w],
but using word_select to extract a single static range would result
in severe bloat of emitted code through expansion to dead branches.
Recognize and simplify this pattern.

nmigen/hdl/ast.py

@@ -304,6 +304,9 @@ class Value(metaclass=ABCMeta):
         Part, out
             Selected part of the ``Value``
         """
+        offset = Value.cast(offset)
+        if type(offset) is Const and isinstance(width, int):
+            return self[offset.value:offset.value + width]
         return Part(self, offset, width, stride=1, src_loc_at=1)
 
     def word_select(self, offset, width):
@@ -324,6 +327,9 @@ class Value(metaclass=ABCMeta):
         Part, out
             Selected part of the ``Value``
         """
+        offset = Value.cast(offset)
+        if type(offset) is Const and isinstance(width, int):
+            return self[offset.value * width:(offset.value + 1) * width]
         return Part(self, offset, width, stride=width, src_loc_at=1)
 
     def matches(self, *patterns):

nmigen/test/test_hdl_ast.py

@@ -533,15 +533,23 @@ class BitSelectTestCase(FHDLTestCase):
 
     def test_shape(self):
         s1 = self.c.bit_select(self.s, 2)
+        self.assertIsInstance(s1, Part)
         self.assertEqual(s1.shape(), unsigned(2))
         self.assertIsInstance(s1.shape(), Shape)
         s2 = self.c.bit_select(self.s, 0)
+        self.assertIsInstance(s2, Part)
         self.assertEqual(s2.shape(), unsigned(0))
 
     def test_stride(self):
         s1 = self.c.bit_select(self.s, 2)
+        self.assertIsInstance(s1, Part)
         self.assertEqual(s1.stride, 1)
 
+    def test_const(self):
+        s1 = self.c.bit_select(1, 2)
+        self.assertIsInstance(s1, Slice)
+        self.assertRepr(s1, """(slice (const 8'd0) 1:3)""")
+
     def test_width_wrong(self):
         with self.assertRaises(TypeError):
             self.c.bit_select(self.s, -1)
@@ -558,13 +566,20 @@ class WordSelectTestCase(FHDLTestCase):
 
     def test_shape(self):
         s1 = self.c.word_select(self.s, 2)
+        self.assertIsInstance(s1, Part)
         self.assertEqual(s1.shape(), unsigned(2))
         self.assertIsInstance(s1.shape(), Shape)
 
     def test_stride(self):
         s1 = self.c.word_select(self.s, 2)
+        self.assertIsInstance(s1, Part)
         self.assertEqual(s1.stride, 2)
 
+    def test_const(self):
+        s1 = self.c.word_select(1, 2)
+        self.assertIsInstance(s1, Slice)
+        self.assertRepr(s1, """(slice (const 8'd0) 2:4)""")
+
     def test_width_wrong(self):
         with self.assertRaises(TypeError):
             self.c.word_select(self.s, 0)