[InstCombine] fold fadd+fneg with fdiv/fmul betweena

[InstCombine] fold fadd+fneg with fdiv/fmul betweena

The backend already does this via isNegatibleForFree(),
but we may want to alter the fneg IR canonicalizations
that currently exist, so we need to try harder to fold
fneg in IR to avoid regressions.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@367227 91177308-0d34-0410-b5e6-96231b3b80d8