x86: zap value-less Disp8MemShift from non-EVEX templates

In order to allow to continue to use templatized SSE2AVX templates when
enhancing those to also cover eGPR usage, Disp8MemShift wants using to
deviate from what general template attributes supply. That requires
using Disp8MemShift in a way also affecting non-EVEX templates, yet
having this attribute set would so far implicitly mean EVEX encoding.
Recognize the case and instead zap the attribute if no other attribute
indicates EVEX encoding.

No change in generated tables.