[Constant] remove fdiv and frem from canTrap()

Assuming the default FP env, we should not treat fdiv and frem any differently in terms of
trapping behavior than any other FP op. Ie, FP ops do not trap with the default FP env.

This matches how we treat the fdiv/frem in IR with isSafeToSpeculativelyExecute() and in 
the backend after:
https://reviews.llvm.org/rL279970

llvm-svn: 279973