Mark tADDrSPi as having side effects again.

It really doesn't, but when r141929 removed the hasSideEffects flag from
this instruction, it caused miscompilations.  I am guessing that it got
moved across a stack pointer update.

Also clear isRematerializable after checking that this instruction is
in fact never rematerialized in the nightly test suite.

llvm-svn: 142030