[LICM] Factor out fault legality from canHoistOrSinkInst [NFC]

This method has three callers, each of which wanted distinct handling:
1) Sinking into a loop is moving an instruction known to execute before a loop into the loop.  We don't need to worry about introducing a fault at all in this case.
2) Hoisting from a loop into a preheader already duplicated the check in the caller.
3) Sinking from the loop into an exit block was the only true user of the code within the routine.  For the moment, this has just been lifted into the caller, but up next is examining the logic more carefully.  Whitelisting of loads and calls - while consistent with the previous code - is rather suspicious.  Either way, a behavior change is worthy of it's own patch.  

llvm-svn: 338671