compiler: fix equally named structs in different scopes

For example, in this code:

    type kv struct {
           v float32
    }

    func foo(a *kv) {
           type kv struct {
                   v byte
           }
    }

Both 'kv' types would be given the same LLVM type, even though they are
different types! This is fixed by only creating a LLVM type once per Go
type (types.Type).

As an added bonus, this change gives a performance improvement of about
0.4%. Not that much, but certainly not nothing for such a small change.

compiler/testdata/basic.ll

@@ -3,6 +3,9 @@ source_filename = "basic.go"
 target datalayout = "e-m:e-p:32:32-i64:64-n32:64-S128"
 target triple = "wasm32--wasi"
 
+%main.kv = type { float }
+%main.kv.0 = type { i8 }
+
 declare noalias nonnull i8* @runtime.alloc(i32, i8*, i8*)
 
 define hidden void @main.init(i8* %context, i8* %parentHandle) unnamed_addr {
@@ -98,3 +101,14 @@ entry:
   %7 = insertvalue { float, float } %6, float %5, 1
   ret { float, float } %7
 }
+
+define hidden void @main.foo(%main.kv* dereferenceable_or_null(4) %a, i8* %context, i8* %parentHandle) unnamed_addr {
+entry:
+  call void @"main.foo$1"(%main.kv.0* null, i8* undef, i8* undef)
+  ret void
+}
+
+define hidden void @"main.foo$1"(%main.kv.0* dereferenceable_or_null(1) %b, i8* %context, i8* %parentHandle) unnamed_addr {
+entry:
+  ret void
+}