Add initial support for "avx1.1" targets for Ivy Bridge.

So far, only the use of the float/half conversion instructions distinguishes
this from the "avx1" target.

Partial work on issue #263.

ispc.h

@@ -207,7 +207,7 @@ struct Target {
         flexible/performant of them will apear last in the enumerant.  Note
         also that __best_available_isa() needs to be updated if ISAs are
         added or the enumerant values are reordered.  */
-    enum ISA { SSE2, SSE4, AVX, AVX2, GENERIC, NUM_ISAS };
+    enum ISA { SSE2, SSE4, AVX, AVX11, AVX2, GENERIC, NUM_ISAS };
 
     /** Instruction set being compiled to. */
     ISA isa;
@@ -252,6 +252,9 @@ struct Target {
         conversions. */
     bool hasHalf;
 
+    /** Indicates whether there is an ISA random number instruciton. */
+    bool hasRand;
+
     /** Indicates whether the target has support for transcendentals (beyond
         sqrt, which we assume that all of them handle). */
     bool hasTranscendentals;