Reapply r294532, reverted in r294787.

Store instructions can have more than one memory operand as a result
of optimizations that fold different stores into one.
When we identify spill instructions to generate DBG_VALUE instructions
to record the spilling of a variable, we disregard stores with 
multiple memory operands for now. We may miss some relevant spills but
the handling is a bit more complex, so we'll do it in a different patch.

This fixes PR31935.

llvm-svn: 295093