[asan] Speed up compilation of large C++ stringmaps (tons of allocas) with ASan

[asan] Speed up compilation of large C++ stringmaps (tons of allocas) with ASan

This addresses PR30746, <https://llvm.org/bugs/show_bug.cgi?id=30746>. The ASan pass iterates over entry-block instructions and checks each alloca whether it's in NonInstrumentedStaticAllocaVec, which is apparently slow. This patch gathers the instructions to move during visitAllocaInst.

Differential Revision: https://reviews.llvm.org/D26380

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@286296 91177308-0d34-0410-b5e6-96231b3b80d8