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 at -O0 so we get this behavior.

Possibly, DAGCombine shouldn't run at all at -O0, but that's for
another day; see PR22346.

Differential Revision: http://reviews.llvm.org/D7181

llvm-svn: 230659