target/mips: Convert MSA 3R instruction format to decodetree (part 1/4)

target/mips: Convert MSA 3R instruction format to decodetree (part 1/4)

Convert 3-register operations to decodetree.

Since the 'data format' field is a constant value, use
tcg_constant_i32() instead of a TCG temporary.

Note, the format definition could be named @3rf_b (for
3R with a df field BYTE-based) but since the instruction
class is named '3R', we simply call the format @3r to
ease reviewing the msa.decode file.
However we directly call the trans_msa_3rf() function,
which handles the BYTE-based df field.

Reviewed-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20211028210843.2120802-21-f4bug@amsat.org>