[GlobalISel] Remove types on selected insts instead of using LLT().

[GlobalISel] Remove types on selected insts instead of using LLT().

LLT() has a particular meaning: it's one invalid type. But we really
want selected instructions to have no type whatsoever.

Also verify that types don't linger after ISel, and enable the verifier
on the AArch64 select test.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@277001 91177308-0d34-0410-b5e6-96231b3b80d8