Rename ScalarType to LaneType.

The word "scalar" is a bit vague and tends to mean "non-vector". Since
we are about to add new CPU flag value types that can't appear as vector
lanes, make the distinction clear: LaneType represents value types that
can appear as a vector lane.

Also replace the Type::is_scalar() method with an is_vector() method.

lib/cretonne/src/isa/intel/abi.rs

@@ -46,7 +46,7 @@ impl ArgAssigner for Args {
 
         // Check for a legal type.
         // We don't support SIMD yet, so break all vectors down.
-        if !ty.is_scalar() {
+        if ty.is_vector() {
             return ValueConversion::VectorSplit.into();
         }