[GlobalISel][AArch64] Split out cast select tests. NFC.

And remove some redundant bitcast tests.

Also split the test functions themselves: it makes it obvious to see
what's tested where and what isn't, it makes the tests much easier to
read and manually update, and, most importantly, it makes them almost
trivial to update using tooling.  Yes, it's obnoxiously verbose, but
said tooling helps upgrade to better MIR syntax whenever available.

llvm-svn: 298222