target/mips: tcg: detect out-of-bounds accesses to cpu_gpr and cpu_gpr_hi

target/mips: tcg: detect out-of-bounds accesses to cpu_gpr and cpu_gpr_hi

In some cases (for example gen_compute_branch_nm in
nanomips_translate.c.inc) registers can be unused
on some paths and a negative value is passed in that case:

        gen_compute_branch_nm(ctx, OPC_BPOSGE32, 4, -1, -2,
                              imm << 1);

To avoid an out of bounds access in those cases, introduce
assertions.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>