[asan] one more workaround for PR17409: don't do BB-level coverage instrumentation if there are more than N (=1500) basic blocks. This makes ASanCoverage work on libjpeg_turbo/jchuff.c used by Chrome, which has 1824 BBs

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206564 91177308-0d34-0410-b5e6-96231b3b80d8