tcg: Consolidate 3 bits into enum TCGTempKind

The temp_fixed, temp_global, temp_local bits are all related.
Combine them into a single enumeration.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>