manual decoding in gen_arith() is not necessary with decodetree. For now
the function is called trans_arith as the original gen_arith still
exists. The former will be renamed to gen_arith as soon as the old
gen_arith can be removed.
Backports commit f2ab1728675772cd475a33f4df3d2f68a22c188f from qemu
gen_arith_imm() does a lot of decoding manually, which was hard to read
in case of the shift instructions and is not necessary anymore with
decodetree.
Backports commit 7a50d3e2ae7f13b24fe55990ea0b8ddcbbb43130 from qemu
we cannot remove the call to gen_arith() in decode_RV32_64G() since it
is used to translate multiply instructions.
Backports commit b73a987b09ad5081123dc6b1e8e6c8305a1c8673 from qemu
for now only LUI & AUIPC are decoded and translated. If decodetree fails, we
fall back to the old decoder.
Backports commit 2a53cff418335ccb4719e9a94fde55f6ebcc895d from qemu