Allow vector types for isplit and iconcat.

These two instructions make sense for vector types by simply performing
the same operation on each lane, like most other vector operations.

Problem found by @angusholder's verifier.

lib/cretonne/meta/base/instructions.py

@@ -1278,8 +1278,8 @@ fcvt_from_sint = Instruction(
 #
 
 WideInt = TypeVar(
-        'WideInt', 'A scalar integer type from `i16` upwards',
-        ints=(16, 64))
+        'WideInt', 'An integer type with lanes from `i16` upwards',
+        ints=(16, 64), simd=True)
 x = Operand('x', WideInt)
 lo = Operand(
         'lo', WideInt.half_width(), 'The low bits of `x`')
@@ -1288,7 +1288,10 @@ hi = Operand(
 
 isplit = Instruction(
         'isplit', r"""
-        Split a scalar integer into low and high parts.
+        Split an integer into low and high parts.
+
+        Vectors of integers are split lane-wise, so the results have the same
+        number of lanes as the input, but the lanes are half the size.
 
         Returns the low half of `x` and the high half of `x` as two independent
         values.
@@ -1297,8 +1300,8 @@ isplit = Instruction(
 
 
 NarrowInt = TypeVar(
-        'NarrowInt', 'A scalar integer type up to `i32`',
-        ints=(8, 32))
+        'NarrowInt', 'An integer type with lanes type to `i32`',
+        ints=(8, 32), simd=True)
 lo = Operand('lo', NarrowInt)
 hi = Operand('hi', NarrowInt)
 a = Operand(
@@ -1308,6 +1311,10 @@ a = Operand(
 iconcat = Instruction(
         'iconcat', r"""
         Concatenate low and high bits to form a larger integer type.
+
+        Vectors of integers are concatenated lane-wise such that the result has
+        the same number of lanes as the inputs, but the lanes are twice the
+        size.
         """,
         ins=(lo, hi), outs=a)