target/ppc: Move cmp{rb, eqb}, tw[i], td[i], isel instructions to decodetree.

target/ppc: Move cmp{rb, eqb}, tw[i], td[i], isel instructions to decodetree.

Moving the following instructions to decodetree specification :

	cmp{rb, eqb}, t{w, d}	: X-form
	t{w, d}i		: D-form
	isel			: A-form

The changes were verified by validating that the tcg ops generated by those
instructions remain the same, which were captured using the '-d in_asm,op' flag.
Also for CMPRB, following review comments :
Replaced repetition of arithmetic right shifting (tcg_gen_shri_i32) followed
by extraction of last 8 bits (tcg_gen_ext8u_i32) with extraction of the required
bits using offsets (tcg_gen_extract_i32).

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Chinmay Rath <rathc@linux.ibm.com>
[np: 32-bit compile fix]
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>