[vm] Let the C++ compiler know it can assume the low bit of a Smi is clear.

TEST=ci
Change-Id: I89f60532a6fe7923a5d4d7bbcd624228ab899854
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/481300
Reviewed-by: Alexander Markov <alexmarkov@google.com>
Commit-Queue: Ryan Macnak <rmacnak@google.com>

runtime/platform/globals.h

@@ -250,6 +250,17 @@ struct simd128_value_t {
 #error Automatic compiler detection failed.
 #endif
 
+#if defined(__clang__)
+#define DART_ASSUME(expr) __builtin_assume(expr)
+#elif defined(__GNUC__)
+#define DART_ASSUME(expr)                                                      \
+  if (!(expr)) __builtin_unreachable()
+#elif defined(_MSC_VER)
+#define DART_ASSUME(expr) __assume(expr)
+#else
+#error Automatic compiler detection failed.
+#endif
+
 #ifdef _MSC_VER
 #elif __GNUC__
 #define DART_HAS_COMPUTED_GOTO 1

runtime/vm/tagged_pointer.h

@@ -457,6 +457,9 @@ inline intptr_t RawSmiValue(const SmiPtr raw_value) {
       static_cast<uint32_t>(static_cast<uintptr_t>(raw_value))));
 #endif
   ASSERT((value & kSmiTagMask) == kSmiTag);
+  // Let the C++ compiler know that it is safe to combine this right shift with
+  // a following left shift, such as often occurs for index scaling.
+  DART_ASSUME((value & kSmiTagMask) == kSmiTag);
   return (value >> kSmiTagShift);
 }