[NFC][InstSimplify] Refactor fminmax-folds.ll test (llvm#160504)

Refactor all the tests in `fminmax-folds.ll` so that they are grouped by
optimization, rather than by intrinsic.

Instead of calling 1 intrinsic per function, each function now tests all
6 variants of the intrinsic. Results are stored to named pointers to
maintain readability in this more compact form. This makes it much
easier to compare the outputs from each intrinsic, rather than having
them scattered in different functions in different parts of the file. It
is also much more compact, so despite adding >50% more tests, the file
is ~500 lines shorter.

The tests added include:
* Adding `maximumnum` and `minimumnum` everywhere (currently not
optimized, but added as a baseline for future optimizations in llvm#139581).
* Adding separate tests for SNaN and QNaN (as a baseline for correctness
improvements in llvm#139581 )
 * Adding tests for scalable vectors
* Increasing the variety of types used in various tests by using more
f16, f64, and vector types in tests.

The only coverage removed is for tests with undef (only poison is now
tested for).

Overall, this refactor should increase coverage, improve readability
with more comments and clear section headers, and make the tests much
more compact and easier to review in llvm#139581 by providing a clear
baseline for each intrinsic's current behaviour.

Author: LewisCrawford