The existing coverage is thin. libcalls.ll seems to be the main fcmp
test, and it doesn't cover all the condition types, and runs with -O0.

Test all conditions for f32 and f64

Adds support for softening FAKE_USE operands.
Adds MSP430 tests that exercise the new softening code.

Fixes #137572

The Target hook convertSelectOfConstantsToMath() needs to be used within
SimplifySelectCC helper combine function in SelectionDAG Isel, where
generic select folding with constants is happening into simple maths op
using the condition as it is.

It necessarily fixes #121145.

Similar to 806761a7629df268c8aed49657aeccffa6bca449

Some targets (e.g. PPC and Hexagon) already did this. I think it's best
to do this consistently so that frontend authors don't run into
inconsistent results when they emit `naked` functions. For example, in
Zig, we had to change our emit code to also set `frame-pointer=none` to
get reliable results across targets.

Note: I don't have commit access.

Targets affected:

- NVPTX and BPF: set to 64 bits.
- ARC, Lanai, and MSP430: set to 0 (they don't implement atomics).

Those which didn't yet add AtomicExpandPass to their pass pipeline now
do so.

This will result in larger atomic operations getting expanded to
`__atomic_*` libcalls via AtomicExpandPass. On all these targets, this
now matches what Clang already does in the frontend.

The only targets which do not configure AtomicExpandPass now are:
- DirectX and SPIRV: they aren't normal backends.
- AVR: a single-cpu architecture with no privileged/user divide, which
could implement all atomics by disabling/enabling interrupts, regardless
of size/alignment. Will be addressed by future work.

This reverts commit 2501ae58e3bb9a70d279a56d7b3a0ed70a8a852c.

Reverted due to various buildbot failures.

PR #66334 tried to renumber slot indexes before register allocation, but
the numbering was still affected by list entries for instructions which
had been erased. Fix this to make the register allocator's live range
length heuristics even less dependent on the history of how instructions
have been added to and removed from SlotIndexes's maps.

This is a follow-up to b71edfaa4ec3c998aadb35255ce2f60bba2940b0
since I forgot the lit.local.cfg files in that one.

Reformatting is done with `black`.

If you end up having problems merging this commit because you
have made changes to a python file, the best way to handle that
is to run git checkout --ours <yourfile> and then reformat it
with black.

If you run into any problems, post to discourse about it and
we will try to help.

RFC Thread below:

https://discourse.llvm.org/t/rfc-document-and-standardize-python-code-style

Reviewed By: barannikov88, kwk

Differential Revision: https://reviews.llvm.org/D150762