AMDGPU/GlobalISel: Fix handling of G_ANYEXT with s1 source

We were letting G_ANYEXT with a vcc register bank through, which was
incorrect and would select to an invalid copy. Fix this up like G_ZEXT
and G_SEXT. Also drop old code to fixup the non-boolean case in
RegBankSelect. We now have to perform that expansion during selection,
so there's no benefit to doing it during RegBankSelect.