[PowerPC][NFC] Rename record instructions to use _rec suffix instead of o

We use o suffix to indicate record form instuctions,
(as it is similar to dot '.' in mne?)

This was fine before, as we did not support XO-form.
However, with https://reviews.llvm.org/D66902,
we now have XO-form support.

It becomes confusing now to still use 'o' for record form,
and it is weird to have something like 'Oo' .

This patch rename all 'o' instructions to use '_rec' instead.
Also rename `isDot` to `isRecordForm`.

Reviewed By: #powerpc, hfinkel, nemanjai, steven.zhang, lkail

Differential Revision: https://reviews.llvm.org/D70758