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

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@45132 91177308-0d34-0410-b5e6-96231b3b80d8