Add a limit to the number of instructions memdep will scan in a single block. ...

Add a limit to the number of instructions memdep will scan in a single block.  This prevents (at least in some cases) O(N^2) runtime in passes like DSE.

The limit in this patch is probably too high, but it is enough to stop DSE from going completely insane on a testcase I have (which has a single block with around 50,000 non-aliasing stores in it).

rdar://9471075

llvm-svn: 133111