asan: do not instrument direct inbounds accesses to stack variables

Do not instrument direct accesses to stack variables that can be
proven to be inbounds, e.g. accesses to fields of structs on stack.

But it eliminates 33% of instrumentation on webrtc/modules_unittests
(number of memory accesses goes down from 290152 to 193998) and
reduces binary size by 15% (from 74M to 64M) and improved compilation time by 6-12%.

The optimization is guarded by asan-opt-stack flag that is off by default.

http://reviews.llvm.org/D7583

llvm-svn: 231241