x86: VCVTNEPS2BF16{X,Y} should permit broadcasting

Just like other VCVT*{X,Y} templates do, and to allow the programmer
flexibility (might be relevant in particular when heavily macro-izing
code), the two templates should also have Broadcast set, just like their
X/Y-suffix-less counterparts. This in turn requires them to also have
* Dword set on their memory operands, to cover the logic added to
  i386gen by 4a1b91ea ("x86: Expand Broadcast to 3 bits"),
* RegXMM/RegYMM set on their source operands, to satisfy broadcast
  sizing logic in gas itself.
Otherwise ATTSyntax templates wouldn't need such operand size attributes.

While extending the test cases, also add Intel syntax broadcast forms
without explicit size specifiers.