refactor: split out an isInductivePredicateVal function (#10033)

This is just like `isInductivePredicate?`, but on an existing
`InductiveVal` rather than one that is looked up by name.

src/Lean/Meta/Basic.lean

@@ -2230,6 +2230,15 @@ def sortFVarIds (fvarIds : Array FVarId) : MetaM (Array FVarId) := do
 
 end Methods
 
+/--
+Return `true` if `indVal` is an inductive predicate. That is, `inductive` type in `Prop`.
+-/
+def isInductivePredicateVal (indVal : InductiveVal) : MetaM Bool := do
+  forallTelescopeReducing indVal.type fun _ type => do
+    match (← whnfD type) with
+    | .sort u .. => return u == levelZero
+    | _ => return false
+
 /--
 Return `some info` if `declName` is an inductive predicate where `info : InductiveVal`.
 That is, `inductive` type in `Prop`.
@@ -2237,10 +2246,10 @@ That is, `inductive` type in `Prop`.
 def isInductivePredicate? (declName : Name) : MetaM (Option InductiveVal) := do
   match (← getEnv).find? declName with
   | some (.inductInfo info) =>
-    forallTelescopeReducing info.type fun _ type => do
-      match (← whnfD type) with
-      | .sort u .. => if u == levelZero then return some info else return none
-      | _ => return none
+    if (← isInductivePredicateVal info) then
+      return some info
+    else
+      return none
   | _ => return none
 
 /-- Return `true` if `declName` is an inductive predicate. That is, `inductive` type in `Prop`. -/