[TableGen] Run GenerateVariants before ExpandHwModeBasedTypes.

A large portion of the patterns are duplicated for HwMode on RISCV.
If we expand HwMode first, we need to check nearly twice as many
patterns for variants. HwModes shouldn't affect whether a variant
is valid so we should be able to expand after.

This also reduces the RISCV isel table by 539 bytes due to factoring
working better on this pattern order. Unfortunately it increases
Hexagon table size by ~50 bytes. But I think this is a reasonable
trade.