x86/Intel: don't accept bogus instructions

x86/Intel: don't accept bogus instructions

... due to their last byte looking like a suffix, when after its
stripping a matching instruction can be found. Since memory operand
size specifiers in Intel mode get converted into suffix representation
internally, we need to keep track of the actual mnemonic suffix which
may have got trimmed off, and check its validity while looking for a
matching template. I tripper over this quite some time again after
support for AMD's SSE5 instructions got removed, as at that point some
of the SSE5 mnemonics, other than expected, didn't fail to assemble.
But the problem affects many more instructions, namely (almost) all
MMX, SSE, and AVX ones as it looks. I don't think it makes sense to
add a testcase covering all of them, nor do I think it makes sense to
pick out some random examples for a new test case.