riscv64: Refactor `Inst::generate_imm`

👋 Hey,

This is a followup from a [comment](https://github.com/bytecodealliance/wasmtime/pull/6915#discussion_r1307848003) from @jameysharp in 6915. `Inst::generate_imm` tries to match a 64 bit constant into a pair of `Imm20` and `Imm12`'s so that we can generate that immediate from a pair of instructions.

The current logic is quite complex and somewhat unreadable, but it doesn't need to be. @jameysharp's version is a lot cleaner, and i suspect is just missing a small fix for a few edge cases. We should clean it up.

This function should be able to generate all 32 bit constants, as well as a good number of 64 bit constants.