[lldb] [Process/FreeBSDRemote] Modernize and simplify YMM logic

Eliminate the remaining swith-case code for register getters,
and migrate YMM registers to regset-oriented model.  Since these
registers are recombined from XMM and YMM_Hi128 XSAVE blocks, while LLDB
gdb-server protocol transmits YMM registers whole, the offset-based
model will not work here.  Nevertheless, some improvement was possible.

Replace generic 'XSaveRegSet' along with sub-sets for XSAVE components
with 'YMMRegSet' (and more regsets in the future as further components
are implemented).  Create a helper GetYMMSplitReg() method that obtains
pointers to the appropriate XMM and YMM_Hi128 blocks to reduce code
duplication.

Differential Revision: https://reviews.llvm.org/D91293