[DSE] Add stat for remaining stores after DSE.

Using the existing NumFastStores statistic can be misleading when
comparing the impact of DSE patches.

For example, consider the case where a store gets removed from a
function before it is inlined into another function. A less
powerful DSE might only remove the store from functions it has
been inlined into, which will result in more stores being removed, but
no difference in the actual number of stores after DSE.

The new stat provides the absolute number of stores surviving after
DSE.

Reviewers: dmgreen, bryant, asbirlea, jfb

Reviewed By: asbirlea

Differential Revision: https://reviews.llvm.org/D78830