HLSL: require tessellation factors to be fixed size arrays

SPIR-V requires that tessellation factor arrays be size 4 (outer) or 2 (inner).
HLSL allows other sizes such as 3, or even scalars.  This commit converts
between them by forcing the IO types to be the SPIR-V size, and allowing
copies between the internal and IO types to handle these cases.

Test/hlsl.hull.ctrlpt-1.tesc

@@ -9,8 +9,8 @@ struct hs_in_t
 
 struct hs_pcf_t
 {
-    float tfactor[3] : SV_TessFactor;
-    float flInFactor : SV_InsideTessFactor;
+    float tfactor[3] : SV_TessFactor; // must turn into a size 4 array in SPIR-V
+    float flInFactor : SV_InsideTessFactor; // must turn into a size 2 array in SPIR-V
 }; 
 
 struct hs_out_t