target/mips: simplify gen_compute_imm_branch logic

target/mips: simplify gen_compute_imm_branch logic

One of the Travis builds was complaining about:

  qemu/include/tcg/tcg.h:437:12: error: ‘cond’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
       return (TCGCond)(c ^ 1);
  ../target/mips/translate.c:20031:13: note: ‘cond’ was declared here
       TCGCond cond;

Rather than figure out exactly which one was causing the complaint I
just defaulted to TCG_COND_ALWAYS and allowed that state to double up
for the now defunct bcond_compute variable.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20200909112742.25730-5-alex.bennee@linaro.org>