Add "mayHaveSideEffects" and "neverHasSideEffects" flags to some instructions. I

based what flag to set on whether it was already marked as
"isRematerializable". If there was a further check to determine if it's "really"
rematerializable, then I marked it as "mayHaveSideEffects" and created a check
in the X86 back-end similar to the remat one.

llvm-svn: 45132