ssa: fix prog.Type error for instantiated generic types

When converting instantiated generic types (e.g., nistCurve[*P256Point]),
the Instantiate function was using types.Instantiate with validate=true.

This caused validation to fail because:
1. cvtNamed creates new type parameters with types.NewTypeParam()
2. The constraint on these new type parameters still references the OLD
   type parameter from the origin type
3. When validating, the method signatures don't match because the
   constraint expects methods with the old type param (e.g., Add(Point, Point))
   but the type argument has methods with concrete types (e.g., Add(*P256Point, *P256Point))

The fix is to use validate=false since the original type was already
validated by the Go type checker.

Fixes #1636

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Co-authored-by: visualfc <714279+visualfc@users.noreply.github.com>

Author: xgopilot

ssa/type_cvt.go

@@ -170,7 +170,10 @@ func Instantiate(orig types.Type, t *types.Named) (types.Type, bool) {
 		for i := 0; i < tp.Len(); i++ {
 			targs[i] = tp.At(i)
 		}
-		if typ, err := types.Instantiate(nil, orig, targs, true); err == nil {
+		// Use validate=false because the newly created type parameters may not
+		// match the original constraint references. The original type was already
+		// validated by the Go type checker, so we can skip validation here.
+		if typ, err := types.Instantiate(nil, orig, targs, false); err == nil {
 			return typ, true
 		}
 	}