Do not use length if we can pattern match (dotnet#5716)

* Do not use length if we can pattern match

* Check field count only once

src/fsharp/NameResolution.fs

@@ -430,8 +430,7 @@ let private GetCSharpStyleIndexedExtensionMembersForTyconRef (amap:Import.Import
             // Method must be static, have 'Extension' attribute, must not be curried, must have at least one argument
             if not minfo.IsInstance && 
                not minfo.IsExtensionMember && 
-               minfo.NumArgs.Length = 1 && 
-               minfo.NumArgs.Head >= 1 && 
+               (match minfo.NumArgs with [x] when x >= 1 -> true | _ -> false) && 
                MethInfoHasAttribute g m g.attrib_ExtensionAttribute minfo
             then
                 let ilExtMem = ILExtMem (tcrefOfStaticClass, minfo, pri)

src/fsharp/TypeChecker.fs

@@ -1941,6 +1941,7 @@ let FreshenAbstractSlot g amap m synTyparDecls absMethInfo =
 let BuildFieldMap cenv env isPartial ty flds m = 
     let ad = env.eAccessRights
     if isNil flds then invalidArg "flds" "BuildFieldMap"
+    let fldCount = flds.Length
 
     let frefSets = 
         let allFields = flds |> List.map (fun ((_, ident), _) -> ident)
@@ -1960,7 +1961,7 @@ let BuildFieldMap cenv env isPartial ty flds m =
                 warning (Error(FSComp.SR.tcFieldsDoNotDetermineUniqueRecordType(), m))
 
             // try finding a record type with the same number of fields as the ones that are given.
-            match tcrefs |> List.tryFind (fun tc -> tc.TrueFieldsAsList.Length = flds.Length) with
+            match tcrefs |> List.tryFind (fun tc -> tc.TrueFieldsAsList.Length = fldCount) with
             | Some tcref -> tcref            
             | _ -> 
                 // OK, there isn't a unique, good type dictated by the intersection for the field refs.