[MC] Remove unneeded MCDataFragment check from AttemptToFoldSymbolOffsetDifference

If FA == FB, we can use SA.getOffset() - SB.getOffset() even if FA is
not a MCDataFragment, as the only case this can be problematic
(different offsets for a variable-size fragment) is invalid/unreachable.
If FA != FB, the `if (FI->getKind() != MCFragment::FT_Data)` check below
can bail out correctly.

This change will help Mach-O fold more expressions. For ELF this is NFC,
unless evaluateFixup has a bug that would evaluate an expression
differently.