Basic support for AVX2 when building with LLVM3.1svn

For now this target just uses the same builtins-*.ll files as the
regular AVX1 target.  Once the gather intrinsic is available from
LLVM, we'll want to have custom target files that call out to that
for gathers. (The integer min/max intrinsics should be wired up to
the __{min,max}_varying_{int,uint}*() builtins at that point as
well.)

ispc.cpp

@@ -161,7 +161,21 @@ Target::GetTarget(const char *arch, const char *cpu, const char *isa,
         t->vectorWidth = 16;
         t->attributes = "+avx,+popcnt,+cmov";
     }
-#endif // LLVM 3.0
+#endif // LLVM 3.0+
+#if defined(LLVM_3_1svn)
+    else if (!strcasecmp(isa, "avx2")) {
+        t->isa = Target::AVX2;
+        t->nativeVectorWidth = 8;
+        t->vectorWidth = 8;
+        t->attributes = "+avx2,+popcnt,+cmov";
+    }
+    else if (!strcasecmp(isa, "avx2-x2")) {
+        t->isa = Target::AVX2;
+        t->nativeVectorWidth = 16;
+        t->vectorWidth = 16;
+        t->attributes = "+avx2,+popcnt,+cmov";
+    }
+#endif // LLVM 3.1
     else {
         fprintf(stderr, "Target ISA \"%s\" is unknown.  Choices are: %s\n", 
                 isa, SupportedTargetISAs());
@@ -201,9 +215,12 @@ Target::SupportedTargetArchs() {
 const char *
 Target::SupportedTargetISAs() {
     return "sse2, sse2-x2, sse4, sse4-x2"
-#if defined(LLVM_3_0) || defined(LLVM_3_0svn) || defined(LLVM_3_1svn)
+#ifndef LLVM_2.9
         ", avx, avx-x2"
-#endif
+#endif !LLVM_2_9
+#ifdef LLVM_3_1svn
+        ", avx2, avx2-x2"
+#endif // LLVM_3_1svn
         ;
 }
 
@@ -281,7 +298,8 @@ Target::GetISAString() const {
         return "sse4";
     case Target::AVX:
         return "avx";
-        break;
+    case Target::AVX2:
+        return "avx2";
     default:
         FATAL("Unhandled target in GetISAString()");
     }