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.

llvm-svn: 38468