[GlobalISel][NFC] Gardening: Make translateSimpleUnaryIntrinsic general

Instead of only having this code work for unary intrinsics, have it work for
an arbitrary number of parameters.

Factor out the cases that fall under this (fma, pow).

This makes it a bit easier to add more intrinsics which don't require any
special work.

Differential Revision: https://reviews.llvm.org/D58079

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@353863 91177308-0d34-0410-b5e6-96231b3b80d8