[lldb] [lit] Split 'register read' tests between zmm*, xmm16+, ymm16+

Since Darwin target implements support for zmm* registers without
matching support for the respectively added xmm* and ymm* registers,
split the tests for each register group.  To reduce code duplication,
the tests are using the same source file (which sets more registers
than necessary but that should not cause any harm).

Differential Revision: https://reviews.llvm.org/D61376

llvm-svn: 359780