[LLVM][LangRef] Remove "n > 0" restriction from get.active.lanes.mask.

The specification for get.active.lanes.mask says a limit value of
zero results in poison. This seems like an artificial restriction and
means you cannot use the intrinsic to create minimal loops of the form:

  foo(int count, ....) {
    int i = 0;
    while (mask = get.active.lane.mask(i, count)) {
      ; do work
      i += count_bits(mask);
    }
  }

I cannot see any code that generates poison in this case, in fact
ConstantFoldFixedVectorCall returns the logical result (i.e. an all
false vector).

There are also cases like `can_overflow_i64_induction_var` in
sve-tail-folding-overflow-checks.ll that look broken by the current
definition? for the case when "%N <= vscale * 4".

For these reasons I'm asking if we can simply remove the restriction?

Author: paulwalker-arm

llvm/docs/LangRef.rst

@@ -24014,8 +24014,7 @@ indexed by ``i``,  and ``%base``, ``%n`` are the two arguments to
 ``llvm.get.active.lane.mask.*``, ``%icmp`` is an integer compare and ``ult``
 the unsigned less-than comparison operator.  Overflow cannot occur in
 ``(%base + i)`` and its comparison against ``%n`` as it is performed in integer
-numbers and not in machine numbers.  If ``%n`` is ``0``, then the result is a
-poison value. The above is equivalent to:
+numbers and not in machine numbers.  The above is equivalent to:
 
 ::
 

llvm/lib/Analysis/Lint.cpp

@@ -374,13 +374,6 @@ void Lint::visitCallBase(CallBase &I) {
       visitMemoryReference(I, MemoryLocation::getForArgument(&I, 0, TLI),
                            std::nullopt, nullptr, MemRef::Read | MemRef::Write);
       break;
-    case Intrinsic::get_active_lane_mask:
-      if (auto *TripCount = dyn_cast<ConstantInt>(I.getArgOperand(1)))
-        Check(!TripCount->isZero(),
-              "get_active_lane_mask: operand #2 "
-              "must be greater than 0",
-              &I);
-      break;
     }
 }