target/mips: Avoid case statements formulated by ranges - part 2

target/mips: Avoid case statements formulated by ranges - part 2

Remove "range style" case statements to make code analysis easier.
This patch handles cases when the values in the range in question
were not properly defined.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Aleksandar Markovic <amarkovic@wavecomp.com>
Signed-off-by: Aleksandar Markovic <amarkovic@wavecomp.com>
Signed-off-by: Stefan Markovic <amarkovic@wavecomp.com>