patch and regression test for problem with vzeroupper

tests/chkstk.ispc

@@ -0,0 +1,49 @@
+//test for 17631 bug in LLVM.
+
+export uniform int width() { return programCount; }
+ 
+struct s_temp
+{
+    float temp[64];
+};
+ 
+int CompressBlockBC7(int A, uniform float b)
+{
+    // This declaration caused problem because LLVM inserted
+    // _chkstk after declaration and vzeroupper before it's call.
+    // A will be in ymm at avx, so we lose a half of it.
+    s_temp _state;
+    // These two loops are here to prevent elimination of declaration
+    for (int i=0; i<64; i++) {
+        float ii = i;
+        _state.temp[i] = b + sin(ii);
+    }
+    float r = 0;
+    for (int j=0; j<64; j+=9) {
+        r += _state.temp[j] + j;
+    }
+
+    // Here upper bits of A in ymm can be zeros. This will crash the test.
+    int B;
+    if (A!=0) {
+        B = 20;
+    }
+    else {
+        B = 30;
+    }
+    if(A == 1) {
+        B = r;
+    }
+    return B;
+}
+ 
+export void f_fu(uniform float RET[], uniform float aFOO[], uniform float b) {
+    int A = programIndex;
+    RET[programIndex] = CompressBlockBC7(A, b);
+}
+ 
+export void result(uniform float RET[]) {
+    RET[programIndex] = 20;
+    RET[0] = 30;
+    RET[1] = 292;
+}