Fix thumbv5e frame lowering assertion failure.

It is possible that frame pointer is not found in the
callee saved info, thus FramePtrSpillFI may be incorrect
if we don't check the result of hasFP(MF).

Besides, if we enable the stack coloring algorithm, there
will be an assertion to ensure the slot is live.  But in
the test case, %var1 is not live in the prologue of the
function, and we will get the assertion failure.

Note: There is similar code in ARMFrameLowering.cpp.