teach instcombine to remove allocated buffers even if there are stores,...

teach instcombine to remove allocated buffers even if there are stores, memcpy/memmove/memset, and objectsize users.
This means we can do cheap DSE for heap memory.
Nothing is done if the pointer excapes or has a load.

The churn in the tests is mostly due to objectsize, since we want to make sure we
don't delete the malloc call before evaluating the objectsize (otherwise it becomes -1/0)

llvm-svn: 159876