move strcascmp on isa to _after_ comparing isa against NULL

ispc.cpp

@@ -478,10 +478,6 @@ Target::Target(const char *arch, const char *cpu, const char *isa, bool pic, boo
         }
     }
 
-    if (!strcasecmp(isa, "host")) {
-        isa = lGetSystemISA();
-    }
-
     if (isa == NULL) {
         // If a CPU was specified explicitly, try to pick the best
         // possible ISA based on that.
@@ -551,6 +547,10 @@ Target::Target(const char *arch, const char *cpu, const char *isa, bool pic, boo
                     isa, cpu);
     }
 
+    if (!strcasecmp(isa, "host")) {
+        isa = lGetSystemISA();
+    }
+
     if (arch == NULL) {
 #ifdef ISPC_ARM_ENABLED
         if (!strncmp(isa, "neon", 4))