Fix constant folding for shl/not

Since the source and destination types are the same the result should be
trimmed to fit.

tests/arithm/tshl.nim

@@ -0,0 +1,34 @@
+discard """
+  output: '''
+0
+0
+1
+1
+0
+0
+0
+1
+'''
+"""
+
+# Signed types
+block:
+  const t0: int8  = 1'i8 shl 8
+  const t1: int16 = 1'i16 shl 16
+  const t2: int32 = 1'i32 shl 32
+  const t3: int64 = 1'i64 shl 64
+  echo t0
+  echo t1
+  echo t2
+  echo t3
+
+# Unsigned types
+block:
+  const t0: uint8  = 1'u8 shl 8
+  const t1: uint16 = 1'u16 shl 16
+  const t2: uint32 = 1'u32 shl 32
+  const t3: uint64 = 1'u64 shl 64
+  echo t0
+  echo t1
+  echo t2
+  echo t3