[IPSCCP] Switch away from Instruction::isSafeToRemove()

In D115737 I found that I needed to teach Instruction::isSafeToRemove()
about strictfp/constrained intrinsics. It was pointed out that this is
probably the wrong function to use isInstructionTriviallyDead(). It doesn't
make sense to have a "second, worse implementation".

I also believe that the Instruction class is the wrong place for this
functionality. The information about whether or not an instruction can be
removed is in the transform passes and should stay there.

Differential Revision: https://reviews.llvm.org/D118387