[AddressSanitizer] Add support for (constant-)masked loads and stores.

This patch adds support for instrumenting masked loads and stores under
ASan, if they have a constant mask.

isInterestingMemoryAccess now supports returning a mask to be applied to
the loads, and instrumentMop will use it to generate additional checks.

Added tests for v4i32 v8i32, and v4p0i32 (~v4i64) for both loads and
stores (as well as a test to verify we don't add checks to non-constant
masks).

Differential Revision: https://reviews.llvm.org/D26230

llvm-svn: 287047