target/mips/op_helper: Convert multiple if() to switch case

target/mips/op_helper: Convert multiple if() to switch case

The cache operation is encoded in bits [20:18] of the instruction.
The 'op' argument of helper_cache() contains the bits [20:16].
Extract the 3 bits and parse them using a switch case. This allow
us to handle multiple cache types (the cache type is encoded in
bits [17:16]).

Previously the if() block was only checking the D-Cache (Primary
Data or Unified Primary). Now we also handle the I-Cache (Primary
Instruction), S-Cache (Secondary) and T-Cache (Terciary).

Reported-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Message-Id: <20200813181527.22551-2-f4bug@amsat.org>