[InstSimplify] add tests for fadd/fsub; NFC

Instcombine gets some, but not all, of these cases via
it's internal reassociation transforms. It fails in
all cases with vector types.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@339168 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed