[llvm] [VPlan] Add opcode to create step for wide inductions. (PR #119284)

[Luke Lau via llvm-commits]

https://github.com/lukel97 commented:

Hi, I was able to use this in #118638 but it looks like there's difference with how this widens the VF + step before multiplying, as opposed to after e.g.:

```diff
--- a/llvm/test/Transforms/LoopVectorize/AArch64/clamped-trip-count.ll
+++ b/llvm/test/Transforms/LoopVectorize/AArch64/clamped-trip-count.ll
@@ -20,9 +20,9 @@ define void @clamped_tc_8(ptr nocapture %dst, i32 %n, i64 %val) vscale_range(1,1
 ; CHECK-NEXT:    [[TMP8:%.*]] = call <vscale x 8 x i64> @llvm.stepvector.nxv8i64()
 ; CHECK-NEXT:    [[TMP7:%.*]] = mul <vscale x 8 x i64> [[TMP8]], splat (i64 1)
 ; CHECK-NEXT:    [[INDUCTION:%.*]] = add <vscale x 8 x i64> zeroinitializer, [[TMP7]]
-; CHECK-NEXT:    [[TMP12:%.*]] = mul i64 1, [[TMP6]]
-; CHECK-NEXT:    [[DOTSPLATINSERT:%.*]] = insertelement <vscale x 8 x i64> poison, i64 [[TMP12]], i64 0
+; CHECK-NEXT:    [[DOTSPLATINSERT:%.*]] = insertelement <vscale x 8 x i64> poison, i64 [[TMP6]], i64 0
 ; CHECK-NEXT:    [[DOTSPLAT:%.*]] = shufflevector <vscale x 8 x i64> [[DOTSPLATINSERT]], <vscale x 8 x i64> poison, <vscale x 8 x i32> zeroinitializer
+; CHECK-NEXT:    [[TMP9:%.*]] = mul <vscale x 8 x i64> [[DOTSPLAT]], splat (i64 1)
 ; CHECK-NEXT:    [[BROADCAST_SPLATINSERT:%.*]] = insertelement <vscale x 8 x i64> poison, i64 [[VAL]], i64 0
 ; CHECK-NEXT:    [[BROADCAST_SPLAT:%.*]] = shufflevector <vscale x 8 x i64> [[BROADCAST_SPLATINSERT]], <vscale x 8 x i64> poison, <vscale x 8 x i32> zeroinitializer
 ; CHECK-NEXT:    br label [[VECTOR_BODY:%.*]]

```

Does this still work if the createWidenCasts are replaced with createScalarCast and then splatted afterwards?

https://github.com/llvm/llvm-project/pull/119284