compiler: do not check for impossible type asserts

Previously there was code to avoid impossible type asserts but it wasn't
great and in fact was too aggressive when combined with reflection.

This commit improves this by checking all types that exist in the
program that may appear in an interface (even struct fields and the
like) but without creating runtime.typecodeID objects with the type
assert. This has two advantages:

  * As mentioned, it optimizes impossible type asserts away.
  * It allows methods on types that were only asserted on (in
    runtime.typeAssert) but never used in an interface to be optimized
    away using GlobalDCE. This may have a cascading effect so that other
    parts of the code can be further optimized.

This sometimes massively improves code size and mostly negates the code
size regression of the previous commit.

transform/testdata/interface.ll

@@ -5,6 +5,8 @@ target triple = "armv7m-none-eabi"
 %runtime.interfaceMethodInfo = type { i8*, i32 }
 
 @"reflect/types.type:basic:uint8" = external constant %runtime.typecodeID
+@"reflect/types.type:basic:uint8$id" = external constant i8
+@"reflect/types.type:basic:int16$id" = external constant i8
 @"reflect/types.type:basic:int" = external constant %runtime.typecodeID
 @"func NeverImplementedMethod()" = external constant i8
 @"Unmatched$interface" = private constant [1 x i8*] [i8* @"func NeverImplementedMethod()"]
@@ -14,9 +16,10 @@ target triple = "armv7m-none-eabi"
 @"reflect/types.type:named:Number" = private constant %runtime.typecodeID { %runtime.typecodeID* @"reflect/types.type:basic:int", i32 0, %runtime.interfaceMethodInfo* getelementptr inbounds ([1 x %runtime.interfaceMethodInfo], [1 x %runtime.interfaceMethodInfo]* @"Number$methodset", i32 0, i32 0) }
 
 declare i1 @runtime.interfaceImplements(i32, i8**)
-declare i1 @runtime.typeAssert(i32, %runtime.typecodeID*)
+declare i1 @runtime.typeAssert(i32, i8*)
 declare i32 @runtime.interfaceMethod(i32, i8**, i8*)
 declare void @runtime.printuint8(i8)
+declare void @runtime.printint16(i16)
 declare void @runtime.printint32(i32)
 declare void @runtime.printptr(i32)
 declare void @runtime.printnl()
@@ -52,7 +55,7 @@ typeswitch.Doubler:
   ret void
 
 typeswitch.notDoubler:
-  %isByte = call i1 @runtime.typeAssert(i32 %typecode, %runtime.typecodeID* nonnull @"reflect/types.type:basic:uint8")
+  %isByte = call i1 @runtime.typeAssert(i32 %typecode, i8* nonnull @"reflect/types.type:basic:uint8$id")
   br i1 %isByte, label %typeswitch.byte, label %typeswitch.notByte
 
 typeswitch.byte:
@@ -62,6 +65,17 @@ typeswitch.byte:
   ret void
 
 typeswitch.notByte:
+  ; this is a type assert that always fails
+  %isInt16 = call i1 @runtime.typeAssert(i32 %typecode, i8* nonnull @"reflect/types.type:basic:int16$id")
+  br i1 %isInt16, label %typeswitch.int16, label %typeswitch.notInt16
+
+typeswitch.int16:
+  %int16 = ptrtoint i8* %value to i16
+  call void @runtime.printint16(i16 %int16)
+  call void @runtime.printnl()
+  ret void
+
+typeswitch.notInt16:
   ret void
 }