[GlobalISel][NFC] Gardening: Factor out code for simple unary intrinsics

There was a lot of repeated code wrt unary math intrinsics in
translateKnownIntrinsic. This factors out the repeated MIRBuilder code into
two functions: translateSimpleUnaryIntrinsic and getSimpleUnaryIntrinsicOpcode.

This simplifies adding simple unary intrinsics, since after this, all you have
to do is add the mapping to SimpleUnaryIntrinsicOpcodes.

Differential Revision: https://reviews.llvm.org/D57774

llvm-svn: 353316