AMDGPU: Fix a use-after free and a missing break

We're erasing MI here, but then immediately using it again inside the
`if`. This moves the erase after we're done using it.

Doing that reveals a second problem though - this case is missing a
break, so we fall through to the default and dereference MI again.
This is obviously a bug, though I don't know how to write a test that
triggers it - all we do in the error case is print some extra debug
output.

Both of these issue crash on lots of tests under ASAN with the
recycling allocator changes from PR26808 applied.

llvm-svn: 264442