target/mips/mxu: Avoid overrun in gen_mxu_q8adde()

target/mips/mxu: Avoid overrun in gen_mxu_q8adde()

Coverity reports a potential overruns (CID 1517770):

  Overrunning array "mxu_gpr" of 15 8-byte elements at
  element index 4294967295 (byte offset 34359738367)
  using index "XRb - 1U" (which evaluates to 4294967295).

Add a gen_extract_mxu_gpr() helper similar to
gen_load_mxu_gpr() to safely extract MXU registers.

Fixes: eb79951ab6 ("target/mips/mxu: Add Q8ADDE ... insns")
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20230712060806.82323-4-philmd@linaro.org>