Convert the pseudo vmsltu.vi/vmsgeu.vi to vmsne.vv/vmseq.vv, when imm is zero.

The original discussion is as follows,
https://github.com/riscv/riscv-v-spec/issues/405

We convert the pseudo instructions

	vmsltu.vi/vmsgeu.vi  vd, vs1, imm

to

	(imm isn't zero) vmsleu.vi/vmsgtu.vi  vd, vs1, imm - 1
	(imm is zero)    vmsne.vv/vmseq.vv    vd, vs1, vs1