portably determine HW SIMD width

[Bruce Carneal]

With dmd you can import core.simd and use a static if/else ladder 
of (is(Vector!(T[SomeSize]))) or is(ubytexx) expressions to 
determine HW SIMD widths.  This doesn't work with ldc because ldc 
is very permissive when it comes to 'SIMD' vector sizes.

I like that about LDC in most contexts but here I'm looking for a 
discriminant rather than an accommodation.  Suggestions?