fixup! [AArch64][llvm] Armv9.7-A: Add support for SVE2p3 LUTI6 operations"

Use `is(AsmToken::LBrac)` when parsing indexed luti6 instructions
of the form: `luti6 { z0.h - z3.h }, { z0.h, z1.h }, { z0, z1 }[0]`

Author: jthackray

llvm/lib/Target/AArch64/AsmParser/AArch64AsmParser.cpp

@@ -4882,7 +4882,7 @@ ParseStatus AArch64AsmParser::tryParseVectorList(OperandVector &Operands,
       FirstReg, Count, Stride, NumElements, ElementWidth, VectorKind, S,
       getLoc(), getContext()));
 
-  if (getTok().isNot(AsmToken::Comma)) {
+  if (getTok().is(AsmToken::LBrac)) {
     ParseStatus Res = tryParseVectorIndex(Operands);
     if (Res.isFailure())
       return ParseStatus::Failure;