[asan] Skip promotable allocas to improve performance at -O0

[asan] Skip promotable allocas to improve performance at -O0

Currently, the ASan executables built with -O0 are unnecessarily slow.
The main reason is that ASan instrumentation pass inserts redundant
checks around promotable allocas. These allocas do not get instrumented
under -O1 because they get converted to virtual registered by mem2reg.
With this patch, ASan instrumentation pass will only instrument non
promotable allocas, giving us a speedup of 39% on a collection of
benchmarks with -O0. (There is no measurable speedup at -O1.)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@230724 91177308-0d34-0410-b5e6-96231b3b80d8