x86: do away with is_evex_encoding()

As we have grown more uses of it, it becomes increasingly more desirable
to replace it by a simpler check. Have i386-gen do at build time what so
far was done at runtime: Deal with templates indicating EVEX-encoding by
other than the EVex attribute, and set that to "dynamic" in such cases.

This then allows simplifying a number of other conditionals as well.