[llvm] [AArch64][CostModel] Increase the cost of illegal SVE int-to-fp converts (PR #130756)

[Sander de Smalen via llvm-commits]

================
@@ -3144,6 +3144,21 @@ InstructionCost AArch64TTIImpl::getCastInstrCost(unsigned Opcode, Type *Dst,
       {ISD::SIGN_EXTEND, MVT::nxv8i32, MVT::nxv8i16, 2},
       {ISD::SIGN_EXTEND, MVT::nxv8i64, MVT::nxv8i16, 6},
       {ISD::SIGN_EXTEND, MVT::nxv4i64, MVT::nxv4i32, 2},
+
+      // Add cost for extending and converting to illegal -too wide- scalable
+      // Extending one size (e.g. i32 -> f64) takes 2 unpacks and 2 fcvts, while
----------------
sdesmalen-arm wrote:

Should the cost of converts of the 'not too wide' types then also be increased to reflect a higher reciprocal cost?
e.g. I see a cost of 1 for:

```
CHECK-NEXT: Cost Model: Found an estimated cost of 1 for instruction: %nv2si16_to_f64 = sitofp <vscale x 2 x i16> undef to <vscale x 2 x double>
```

https://github.com/llvm/llvm-project/pull/130756