Revise Usual Arithmetic Conversions (#311)

There are a few discrepancies between what I originally wrote here and
what DXC does. These changes adjust to the actual behavior and more
closely align with implicit conversion sequences.

Fixes #308

specs/language/expressions.tex

@@ -27,14 +27,17 @@
 \begin{itemize}
   \item If either operand is of scoped enumeration type no conversion is
   performed, and the expression is ill-formed if the types do not match.
-  \item If either operand is a \texttt{vector<T,X>}, vector extension is
-  performed with the following rules:
+  \item If either operand is a \texttt{vector<T,X>}, vector truncation or scalar
+  extension is performed with the following rules:
   \begin{itemize}
-    \item If both vectors are of the same length, no extension is required.
+    \item If both vectors are of the same length, no dimension conversion is
+    required.
     \item If one operand is a vector and the other operand is a scalar, the
-    scalar is extended to a vector via a Splat conversion (\ref{Conv.vsplat}).
+    scalar is extended to a vector via a Splat conversion (\ref{Conv.vsplat}) to
+    match the length of the vector.
     \item Otherwise, if both operands are vectors of different lengths, the
-    expression is ill-formed.
+    vector of longer length is truncated to match the length of the shorter
+    vector (\ref{Conv.vtrunc}).
   \end{itemize}
   \item If either operand is of type \texttt{double} or \texttt{vector<double,
   X>}, the other operator shall be converted to match.