[DAG][X86] Convert isNegatibleForFree/GetNegatedExpression to a target hook (PR42863)

This patch converts the DAGCombine isNegatibleForFree/GetNegatedExpression into overridable TLI hooks.

The intention is to let us extend existing FNEG combines to work more generally with negatible float ops, allowing it work with target specific combines and opcodes (e.g. X86's FMA variants).

Unlike the SimplifyDemandedBits, we can't just handle target nodes through a Target callback, we need to do this as an override to allow targets to handle generic opcodes as well. This does mean that the target implementations has to duplicate some checks (recursion depth etc.).

Partial reversion of rL372756 - I've identified the infinite loop issue inside the X86 override but haven't fixed it yet so I've only (re)committed the common TargetLowering refactoring part of the patch.

Differential Revision: https://reviews.llvm.org/D67557

llvm-svn: 373343