Clear unknown mem ops when merging stack slots (pr14090)

When merging stack slots, if StackColoring::remapInstructions gets a
value back from GetUnderlyingObject that it does not know about or is
not itself a stack slot, clear the memory operand in case it aliases
the merged slot. This prevents the introduction of incorrect aliasing
information.

Author:    Matthew Curtis <mcurtis@codeaurora.org>
llvm-svn: 166216