[lldb][AArch64] Invalidate cached VG value before reconfiguring SVE registers

This fixes 46b961f3.

Prior to the SME changes the steps were:
* Invalidate all registers.
* Update value of VG and use that to reconfigure the registers.
* Invalidate all registers again.

With the changes for SME I removed the initial invalidate thinking
that it didn't make sense to do if we were going to invalidate them
all anyway after reconfiguring.

Well the reason it made sense was that it forced us to get the
latest value of vg which we needed to reconfigure properly.

Not doing so caused a test failure on our Graviton bot which has SVE
(https://lab.llvm.org/buildbot/#/builders/96/builds/45722). It was
flaky and looping it locally would always fail within a few minutes.
Presumably it was using an invalid value of vg, which caused some offsets
to be calculated incorrectly.

To fix this I've invalided vg in AArch64Reconfigure just before we read
it. This is the same as the fix I have in review for SME's svg register.

Pushing this directly to fix the ongoing test failure.