[X86] Merge negated ISD::SUB nodes into X86ISD::SUB equivalent (PR40483)

Follow up to D58597, where it was noted that the commuted ISD::SUB variant
was having problems with lack of combines.

See also D63958 where we untangled setcc/sub pairs.

Differential Revision: https://reviews.llvm.org/D58875

llvm-svn: 365791