MIPS: Support more cases with alien mode of SHF.DF

Currently, we support the cases that strictly fit for the instructions.
For example, for V16QImode, we only support shuffle like
(0<=N0, N1, N2, N3<=3 here)
	N0,	N1,	N2,	N3
	N0+4	N1+4	N2+4,	N3+4
	N0+8	N1+8	N2+8,	N3+8
	N0+12	N1+12	N2+12,	N3+12

While in fact we can support more cases to try use other SHF.DF
instructions not strictly fitting the mode.

1) We can use SHF.H to support more cases for V16QImode:
(M0/M1/M2/M3 are 0 or 2 or 4 or 6)
	M0	M0+1,	M1,	M1+1
	M2	M2+1,	M3,	M3+1
	M0+8	M0+9,	M1+8,	M1+9
	M2+8	M2+9,	M3+8,	M3+9

2) We can use SHF.W to support some cases for V16QImode:
(M0/M1/M2/M3 are 0 or 4 or 8 or 12)
	M0,	M0+1,	M0+2,	M0+3
	M1,	M1+1,	M1+2,	M1+3
	M2,	M2+1,	M2+2,	M2+3
	M3,	M3+1,	M3+2,	M3+3

3) We can use SHF.W to support some cases for V8HImode:
(M0/M1/M2/M3 are 0 or 2 or 4 or 6)
	M0,	M0+1
	M1,	M1+1
	M2,	M2+1
	M3,	M3+1

4) We can also use SHF.W to swap the 2 parts of V2DF or V2DI.

gcc
	* config/mips/mips-protos.h: New function mips_msa_shf_i8.
	* config/mips/mips-msa.md(MSA_WHB_W): Not used anymore;
	(msa_shf_<msafmt_f>): Use mips_msa_shf_i8.
	* config/mips/mips.cc(mips_const_vector_shuffle_set_p):
	Support more cases try to use alien mode instruction;
	(mips_msa_shf_i8): New function to get the correct MSA SHF
	instruction and IMM.