[GlobalISel][CallLowering] Use hasRetAttr for return flags on CallBases

Given something like this:

```
declare signext i16 @signext_callee()
define i32 @caller() {
  %res = call i16 @signext_callee()
  ...
}
```

CallLowering would miss that signext_callee's return value is sign extended,
because it isn't on the call.

Use hasRetAttr on the CallBase to allow us to catch this.

(This now inserts G_ASSERT_SEXT/G_ASSERT_ZEXT like in the original review.)

Differential Revision: https://reviews.llvm.org/D86228