Add a new LargeBlockInfo helper, which is just a wrapper around

a trivial dense map.  Use this in RewriteSingleStoreAlloca to
avoid aggressively rescanning blocks over and over again.  This
fixes PR2925, speeding up mem2reg on the testcase in that bug
from 4.56s to 0.02s in a debug build on my machine.

llvm-svn: 58227