[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

Author: Simon Moll

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -5349,8 +5349,9 @@ LoopVectorizationCostModel::computeFeasibleMaxVF(unsigned ConstTripCount) {
   LLVM_DEBUG(dbgs() << "LV: The Widest register safe to use is: "
                     << WidestRegister << " bits.\n");
 
-  assert(MaxVectorSize <= 256 && "Did not expect to pack so many elements"
-                                 " into one vector!");
+  assert(MaxVectorSize <= WidestRegister &&
+         "Did not expect to pack so many elements"
+         " into one vector!");
   if (MaxVectorSize == 0) {
     LLVM_DEBUG(dbgs() << "LV: The target has no vector registers.\n");
     MaxVectorSize = 1;