[LV][NFC-ish] Allow vector widths over 256 elements

The assertion that vector widths are <= 256 elements was hard wired in the LV code. Eg, VE allows for vectors up to 512 elements. Test again the TTI vector register bit width instead - this is an NFC for non-asserting builds.

Reviewed By: fhahn

Differential Revision: https://reviews.llvm.org/D91518