[llvm] [AArch64] Enable maximize vector bandwidth for Neoverse-V1, V2 and N1 (PR #166748)

[Sam Tebbs via llvm-commits]

================
@@ -50,7 +50,7 @@ define i64 @test_external_iv_user(ptr %a, ptr %b) #0 {
 ; CHECK-NEXT: Cost of 0 for VF 16: induction instruction   %i.iv = phi i64 [ 0, %entry ], [ %i.iv.next, %for.body ]
 ; CHECK-NEXT: Cost of 0 for VF 16: EMIT vp<{{.+}}> = CANONICAL-INDUCTION ir<0>, vp<%index.next>
 ; CHECK: Cost for VF 16: 57
-; CHECK: LV: Selecting VF: vscale x 2
+; CHECK: LV: Selecting VF: 16
----------------
SamTebbs33 wrote:

The VF chosen for fully-unrolled-cost.ll has changed to a fixed-width VF
because of how clampVFByMaxTripCount is called when max vector bandwidth
is called, specifically if FoldTailByMasking is false in this snippet:

```
    return ElementCount::get(ClampedUpperTripCount,
                             FoldTailByMasking ? VF.isScalable() : false);
```

https://github.com/llvm/llvm-project/pull/166748