This cleans up @l @ha optimization in PPCAsmParser and is also the first
step toward removing VK_PPC_* from the generic MCSymbolRefExpr::VariantKind.
Basically we ensure that @l @ha family modifiers always lead to
PPCMCExpr and avoid MCSymbolRefExpr::VariantKind. This allows us
to delete a lot of switch statements that involve a long list of VK_PPC_LO/VK_PPC_HI/...
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