s390x: misc. fixes pertaining to vector insns

s390x: misc. fixes pertaining to vector insns

This patch is fall-out from working on fixing the disassembly for
the vector insns.

Specifically:
1) replace vassert with s390_insn_assert where appropriate
2) add missing s390_insn_asserts (many)
3) remove incorrect s390_insn_asserts (few)
4) check availability of vector opcodes based on hardware capabilities
   and issue an emulation failure if opcode is not available
5) fix a typo in a mnemonic: vfspo -> vfpso
6) inline function s390_vr_get_n_elem

Part of fixing https://bugs.kde.org/show_bug.cgi?id=495817