[llvm] [LV] Consider whether vscale is a known power of two for iteration check (PR #144963)

[Philip Reames via llvm-commits]

preames wrote:

> > I don't believe this is possible to test on a target without scalable vector support. I tried, but the costs returned are all invalid, and thus we refuse to vectorize entirely.
> 
> Does `-force-target-instruction-cost=1` help?

Nope, I continue to get "remark: <unknown>:0:0: UserVF ignored because of invalid costs.".

My attempt in case anyone has suggestions:
```
; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
; RUN: opt -passes=loop-vectorize -force-tail-folding-style=data -prefer-predicate-over-epilogue=predicate-dont-vectorize -force-target-supports-scalable-vectors -force-vector-width=4 -force-target-instruction-cost=1 -scalable-vectorization=on -S < %s | FileCheck %s

; The target does not support predicated vectorization.
define void @foo(ptr noalias %a, ptr noalias %b, ptr noalias %c, i64 %N) {
; CHECK-LABEL: @foo(
; CHECK-NEXT:  entry:
; CHECK-NEXT:    br label [[FOR_BODY:%.*]]
; CHECK:       for.body:
; CHECK-NEXT:    [[IV:%.*]] = phi i64 [ 0, [[ENTRY:%.*]] ], [ [[IV_NEXT:%.*]], [[FOR_BODY]] ]
; CHECK-NEXT:    [[ARRAYIDX:%.*]] = getelementptr inbounds i32, ptr [[B:%.*]], i64 [[IV]]
; CHECK-NEXT:    [[TMP0:%.*]] = load i32, ptr [[ARRAYIDX]], align 4
; CHECK-NEXT:    [[ARRAYIDX2:%.*]] = getelementptr inbounds i32, ptr [[C:%.*]], i64 [[IV]]
; CHECK-NEXT:    [[TMP1:%.*]] = load i32, ptr [[ARRAYIDX2]], align 4
; CHECK-NEXT:    [[ADD:%.*]] = add nsw i32 [[TMP1]], [[TMP0]]
; CHECK-NEXT:    [[ARRAYIDX4:%.*]] = getelementptr inbounds i32, ptr [[A:%.*]], i64 [[IV]]
; CHECK-NEXT:    store i32 [[ADD]], ptr [[ARRAYIDX4]], align 4
; CHECK-NEXT:    [[IV_NEXT]] = add nuw nsw i64 [[IV]], 1
; CHECK-NEXT:    [[EXITCOND_NOT:%.*]] = icmp eq i64 [[IV_NEXT]], [[N:%.*]]
; CHECK-NEXT:    br i1 [[EXITCOND_NOT]], label [[FOR_COND_CLEANUP:%.*]], label [[FOR_BODY]]
; CHECK:       for.cond.cleanup:
; CHECK-NEXT:    ret void
;
entry:
  br label %for.body

for.body:
  %iv = phi i64 [ 0, %entry ], [ %iv.next, %for.body ]
  %arrayidx = getelementptr inbounds i32, ptr %b, i64 %iv
  %0 = load i32, ptr %arrayidx, align 4
  %arrayidx2 = getelementptr inbounds i32, ptr %c, i64 %iv
  %1 = load i32, ptr %arrayidx2, align 4
  %add = add nsw i32 %1, %0
  %arrayidx4 = getelementptr inbounds i32, ptr %a, i64 %iv
  store i32 %add, ptr %arrayidx4, align 4
  %iv.next = add nuw nsw i64 %iv, 1
  %exitcond.not = icmp eq i64 %iv.next, %N
  br i1 %exitcond.not, label %for.cond.cleanup, label %for.body

for.cond.cleanup:
  ret void
}

```

https://github.com/llvm/llvm-project/pull/144963