[SM6.9] Fix LV cast crash

[V-FEXrt]

Fixes #7915

There is a detailed writeup in the issue but in summary, native vectors in SM6.9 shouldn't be optimized in the way this path enables and weren't cleanly being rejected. This change now rejects them

[tex3d]

I mentioned in the issue here:
I don't understand why vectors should come into play at all, unless there's another bug. Why would you expect to add up the mantissa bits from multiple scalars in a vector to see if it's representable by a scalar int type? Am I missing something?

In the issue:

MinBW for a float2 is 65 so it fails the check and early exits as expected.

But what if it were half? Why is the vector size relevant at all for what should be a scalar transformation?

[V-FEXrt]

But what if it were half?

For half2 both with and without -enable-16bit-types MinBW is 66 (turns out float2 is also 66 and I had a typo initially)