[asan] Remove an unsigned compare >= 0

This is not needed after we've forked the Myriad version.  Not to
mention it produces a compiler warning.

Differential Revision: https://reviews.llvm.org/D47054

llvm-svn: 332744

compiler-rt/lib/asan/asan_mapping.h

@@ -307,7 +307,7 @@ namespace __asan {
 
 static inline bool AddrIsInLowMem(uptr a) {
   PROFILE_ASAN_MAPPING();
-  return a >= kLowMemBeg && a <= kLowMemEnd;
+  return a <= kLowMemEnd;
 }
 
 static inline bool AddrIsInLowShadow(uptr a) {