GlobalISel: account for differing exception selector sizes.

For some reason the exception selector register must be a pointer (that's
assumed by SDag); on the other hand, it gets moved into an IR-level type which
might be entirely different (i32 on AArch64). IRTranslator needs to be aware of
this.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@293546 91177308-0d34-0410-b5e6-96231b3b80d8