This converts all ptr element shuffle vectors to s64, so that the
existing vector legalization handling can lower them as needed. This
prevents a lot of fallbacks that currently try to generate things like
`<2 x ptr> G_EXT`.
I'm not sure if bitcast/inttoptr/ptrtoint is intended to be necessary
for vectors of pointers, but it uses buildCast for the casts, which now
generates a ptrtoint/inttoptr.
d3ce069572