[InstCombine] fold fadd+fsub with common operand

This is a sibling to the simplify from:
https://reviews.llvm.org/rL339174


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@339267 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed