[AArch64] Re-enable rematerialization for streaming-mode-changing functions. (#83235)

We can add implicit defs/uses of the 'VG' register to the instructions
to prevent the register allocator from rematerializing values in between
streaming-mode changes, as the def/use of VG will further nail down the
ordering that comes out of ISel. This avoids the heavy-handed approach
to prevent any kind of rematerialization.

While we could add 'VG' as a Use to all SVE instructions, we only really
need to do this for instructions that are rematerializable, as the
smstart/smstop instructions and pseudos act as scheduling barriers which
is sufficient to prevent other instructions from being scheduled in
between the streaming-mode-changing call sequence. However, we may
revisit this in the future.