Initialize the IndexedModeActions array with memset before

Initialize the IndexedModeActions array with memset before
updating it with calls to setIndexedLoadAction/setIndexedStoreAction,
which only update a few bits at a time. This avoids ostensible
undefined behavior of operationg on values which may be
trap-representations, and as a practical matter fixes errors from
valgrind, which doesn't track uninitialized memory with bit
granularity.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@38468 91177308-0d34-0410-b5e6-96231b3b80d8