[PATCH,RS6000 0/5] Clean up MASK_ and RS6000_BTM_ defines

[will schmidt <will_schmidt@vnet.ibm.com>]

Hi,
  This series cleans up the assorted MASK_<xxxx>, OPTION_MASK_<xxxx>,
and RS6000_BTM_<xxxx> defines that we have sprinkled through the
rs6000 target code.

The MASK_ entries are currently defined as their OPTION_MASK_
equivalents since their introduction when the rs6000_isa_flags was
added via commit 4d9675496a28ef6184f2a9c3ac5e6e3ea63606c1 .
This series replaces references to the MASK_ entries with their
OPTION_MASK equivalents as much as possible.

The RS6000_BTM_ defines are mostly unused since the built-in rewrites
from late 2021 and early 2022, and the remaining usage is
straightforward to replace with OPTION_MASK_ values.

The OPTION_MASK_ definitions themselves remain.

Due to size and to keep some of these changes clean I have split this
into several parts.

After this series there are a few remaining MASK_<xxxx> entries
(MASK_POWERPC64, MASK_64BIT and MASK_LITTLE_ENDIAN) which are
conditionally defined, and potentially more invasive to resolve.
Those are deliberately not addressed as part of this series.

This has cleanly regtested (no functional change).  When approved
this series will be committed as a group, though it should be
bisectable.

OK for trunk?

    1/5: Remove unused defines and touch up comments.
    2/5: Rework RS6000_BTM_foo defines, part 1.
    3/5: Rework RS6000_BTM_foo defines, part 2.
    4/5: Rework MASK_foo defines, part 1.
    5/5. Rework MASK_foo defines, part 2.