[asan] Avoid assert failure for non-default shadow scale

[waltl]

Summary

Rather than assertion failing, we can fall back to the
non-optimized version which works for any shadow scale.

compiler-rt/lib/asan/asan_fake_stack.cc

 static const u64 kMagic4 = (kMagic2 << 16) | kMagic2;
 static const u64 kMagic8 = (kMagic4 << 32) | kMagic4;
 
 static const u64 kAllocaRedzoneSize = 32UL;
 static const u64 kAllocaRedzoneMask = 31UL;
 
 // For small size classes inline PoisonShadow for better performance.
 ALWAYS_INLINE void SetShadow(uptr ptr, uptr size, uptr class_id, u64 magic) {
-  CHECK_EQ(SHADOW_SCALE, 3);  // This code expects SHADOW_SCALE=3.
   u64 *shadow = reinterpret_cast<u64*>(MemToShadow(ptr));
-  if (class_id <= 6) {
+  if (SHADOW_SCALE == 3 && class_id <= 6) {
+    // This code expects SHADOW_SCALE=3.
     for (uptr i = 0; i < (((uptr)1) << class_id); i++) {
       shadow[i] = magic;
       // Make sure this does not become memset.
       SanitizerBreakOptimization(nullptr);
     }
   } else {
     // The size class is too big, it's cheaper to poison only size bytes.
     PoisonShadow(ptr, size, static_cast<u8>(magic));