[LSR] Recognize vscale-relative immediates (llvm#88124)

Extends LoopStrengthReduce to recognize immediates multiplied by vscale, and query the current target for whether they are legal offsets for memory operations or adds.

Author: huntergr-arm

llvm/include/llvm/Support/TypeSize.h

@@ -170,6 +170,9 @@ template <typename LeafTy, typename ValueTy> class FixedOrScalableQuantity {
   /// Returns whether the quantity is scaled by a runtime quantity (vscale).
   constexpr bool isScalable() const { return Scalable; }
 
+  /// Returns true if the quantity is not scaled by vscale.
+  constexpr bool isFixed() const { return !Scalable; }
+
   /// A return value of true indicates we know at compile time that the number
   /// of elements (vscale * Min) is definitely even. However, returning false
   /// does not guarantee that the total number of elements is odd.