[InstCombine] move shuffle after fma with same-shuffled operands

https://alive2.llvm.org/ce/z/sD-JVv

This extends 432c199e with a 3 arg intrinsic to demonstrate
that the code works with the extra operand.

Eventually, we will want to use llvm::isTriviallyVectorizable()
or create some new API for this list, but for now, I am intentionally
making a minimum change to reduce risk and only affect an intrinsic
with regression tests in place.