[InstCombine] add tests for fsub folds; NFC

The scalar cases are handled in instcombine's internal
reassociation pass for FP ops, but it misses the vector types.

These patterns are similar to what was handled in InstSimplify in:
https://reviews.llvm.org/rL339171
https://reviews.llvm.org/rL339174
https://reviews.llvm.org/rL339176
...but we can't use instsimplify on these because we require negation
of the original operand.

llvm-svn: 339263