[FNeg] Add FNeg Instruction to LangRef document

The FNeg IR Instruction code was added with D53877.

Differential Revision: https://reviews.llvm.org/D54549



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@347086 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed