target/riscv: progressively load the instruction during decode

target/riscv: progressively load the instruction during decode

The plugin system would throw up a harmless warning when it detected
that a disassembly of an instruction didn't use all it's bytes. Fix
the riscv decoder to only load the instruction bytes it needs as it
needs them.

This drops opcode from the ctx in favour if passing the appropriately
sized opcode down a few levels of the decode.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Robert Foley <robert.foley@linaro.org>
Message-Id: <20200225124710.14152-15-alex.bennee@linaro.org>