[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.

Author: MaskRay

llvm/lib/MC/MCExpr.cpp

@@ -651,8 +651,6 @@ static void AttemptToFoldSymbolOffsetDifference(
     // this is important when the Subtarget is changed and a new MCDataFragment
     // is created in the case of foo: instr; .arch_extension ext; instr .if . -
     // foo.
-    if (FA != FB && (!isa<MCDataFragment>(FA) || !isa<MCDataFragment>(FA)))
-      return;
     if (SA.isVariable() || SB.isVariable() ||
         FA->getSubsectionNumber() != FB->getSubsectionNumber())
       return;