Implement support for constructing a 2x2 matrix from a vector.

BUG=380353

Change-Id: Ica0b94104e39f87b4a3439df40221b1f682fd678
Reviewed-on: https://chromium-review.googlesource.com/203293
Tested-by: Nicolas Capens <nicolascapens@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shannon Woods <shannonwoods@chromium.org>
diff --git a/src/compiler/translator/OutputHLSL.cpp b/src/compiler/translator/OutputHLSL.cpp
index 4e94103..dc79c51 100644
--- a/src/compiler/translator/OutputHLSL.cpp
+++ b/src/compiler/translator/OutputHLSL.cpp
@@ -3590,7 +3590,12 @@
                 }
             }
         }
-        else UNREACHABLE();
+        else
+        {
+            ASSERT(rows == 2 && cols == 2 && parameter.isVector() && parameter.getNominalSize() == 4);
+
+            constructor += "x0";
+        }
     }
     else
     {