[asan] Speed up isInterestingAlloca check

We make many redundant calls to isInterestingAlloca in the AddressSanitzier
pass. This is especially inefficient for allocas that have many uses. Let's
cache the results to speed up compilation.

The compile time improvements depend on the input. I did not see much
difference on benchmarks; however, I have a test case where compile time
goes from minutes to under a second.

llvm-svn: 233397