[DAGCombiner] Avoid combining adjacent stores at -O0 to improve debug experience

When the source has a series of assignments, users reasonably want to
have the debugger step through each one individually. Turn off the combine
for adjacent stores so we get this behavior at -O0.

Similar to D7181.

Reviewed By: spatel, xgupta

Differential Revision: https://reviews.llvm.org/D115808