Cleanup handling of constant function casts.

Some of this code is no longer necessary since int<->ptr casts are no
longer occur as of r187444.

This also fixes handling vectors of pointers, and adds a bunch of new
testcases for vectors and address spaces.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@190885 91177308-0d34-0410-b5e6-96231b3b80d8