[AArch64][GlobalISel] Split legalizer combine tests. NFC.

Big functions with large vreg # are quite unwieldy to update.  This test
also relied on legal s8 operations which we're considering removing.

Change it to have one function per test (it does increase boilerplate,
but makes the core hopefully more readable and maintanable), and use
100% legal operations throughout.

llvm-svn: 292551