[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