Question about LLVM r184574

Hi Andrew,

While working on the MSP430 backend, I noticed code that compiled fine before
hitting an assert which you have recently inserted in r184574. More
specifically, in InlineSpiller.cpp:1076 the following assert is triggered:

assert(MO->isDead() && "Cannot fold physreg def");

I wouldn't be surprised is the underlying cause is in the MSP430 backend but I
wanted to consult you before diving into the code.

Is this assert really necessary? If I change to code to simply "continue" on
"!MO->isDead()", everything seems to be fine and all testcases still pass.