[MC] Simplify the logic of applying fixup for fragments, NFCI
Replace mutiple if else clauses with a switch clause and remove redundant checks. Before this patch, we need to add a statement like if(!isa<MCxxxFragment>(Frag)) here each time we add a new kind of MCEncodedFragment even if it has no fixups. After this patch, we don't need to do that.
Reviewed By: MaskRay
Differential Revision: https://reviews.llvm.org/D83366