all: VBMI2 instructions (#363)

Adds the "Vector Bit Manipulation Instructions 2" instruction set.

These new instructions are added via the `opcodesextra` mechanism #345, since
they're missing from the opcodes database.

Contributed by @vsivsi. Extracted from #349 with simplifications.
Specifically, as prompted by the `dupl` linter we extract some common forms
lists into a helper `forms.go` file.

Co-authored-by: Vaughn Iverson <vsivsi@yahoo.com>