[llvm] [LV][NFC] Factor out MinBWs of values from the cost model (PR #194492)

[David Sherwood via llvm-commits]

================
@@ -563,6 +563,15 @@ bool VFSelectionContext::runtimeChecksRequired() {
   return false;
 }
 
+const MapVector<Instruction *, uint64_t> &
+VFSelectionContext::getMinimalBitwidths() {
+  if (!MinBWsComputed) {
+    MinBWs = computeMinimumValueSizes(TheLoop->getBlocks(), *DB, &TTI);
----------------
david-arm wrote:

Can this initialisation be done in the constructor for VFSelectionContext similar to what we do for VScaleForTuning, e.g.

```
  VFSelectionContext(const TargetTransformInfo &TTI,
                     const LoopVectorizationLegality *Legal,
                     const Loop *TheLoop, const Function &F,
                     PredicatedScalarEvolution &PSE,
                     OptimizationRemarkEmitter *ORE,
                     const LoopVectorizeHints *Hints, bool OptForSize)
      : TTI(TTI), Legal(Legal), TheLoop(TheLoop), F(F), PSE(PSE), ORE(ORE),
        Hints(Hints),
        CostKind(F.hasMinSize() ? TTI::TCK_CodeSize : TTI::TCK_RecipThroughput),
        OptForSize(OptForSize) {
    initializeVScaleForTuning();
  }
```

That avoids having to compute this lazily and you can remove the `MinBWsComputed` flag.

https://github.com/llvm/llvm-project/pull/194492