make @fastmath mirror how lowering applies literal_pow

[KlausC]

The expressions a^x and @fastmath a^x are now producing equivalent results (apart from floating point accuracy) in the case of literal integer x.
The logic in the fastmath macro, trying to mimic the behaviour of the compiler is fixed.

Fixes #53817

[Seelengrab]

Per CONTRIBUTING.md, please provide a good description of what the PR does:

Descriptive commit messages are good.

It's very cumbersome to have to go through three layers of indirection when looking at the history of a file (the change itself, this PR, and finally the issue itself) to figure out what exactly was broken with @fastmath in conjunction with literal_pow.

[KlausC]

It's very cumbersome to have to go through three layers of indirection

Sorry for the inconvenience - I wanted to avoid repeating the error description for the issue. I added a description of what has been changed.

[Seelengrab]

Thank you!

[oscardssmith]

LGTM, although it would probably be good to add performance results to the PR, and ideally an example to the base benchmarks so we can see if this regresses.

[KlausC]

add performance results to the PR

No performance changes are expected during runtime, because only the macro logic is changed.

The cases, which behave different do not exhibit changed performance figures. The 4th example in the changed
version now throws DomainError, like the corresponding non-fastmath version.

For the cases, which now create different runtime code:

NEW: Version 1.12.0-DEV.233 (2024-03-22) krc/fastmath-literalpow/8615c0bdff

julia> foop1(a) = @fastmath a ^ 2305843009213693951
foop1 (generic function with 1 method)

julia> foop2(a) = @fastmath a ^ 2305843009213693952
foop2 (generic function with 1 method)

julia> foom2(a) = @fastmath a ^ -2305843009213693952
foom2 (generic function with 1 method)

julia> foom3(a) = @fastmath a ^ -2305843009213693953
foom3 (generic function with 1 method)

julia> a = 2
2

julia> @btime foop1($a)
  113.349 ns (0 allocations: 0 bytes)
0

julia> @btime foop2($a)
  24.203 ns (0 allocations: 0 bytes)
0

julia> @btime foom2($a)
  209.300 ns (0 allocations: 0 bytes)
0.0

julia> @btime foom3($a)
ERROR: DomainError with -2305843009213693953:

OLD: Julia Version 1.12.0-DEV.225

julia> @btime foop1($a)
  111.230 ns (0 allocations: 0 bytes)
0

julia> @btime foop2($a)
  24.192 ns (0 allocations: 0 bytes)
0

julia> @btime foom2($a)
  209.556 ns (0 allocations: 0 bytes)
0.0

julia> @btime foom3($a)
  209.655 ns (0 allocations: 0 bytes)
0.0

[oscardssmith]

Looks good to me.

[mbauman]

My only hesitation — both here and in #53713 is that this really seems to be a bug in the Julia-flisp interface that doesn't match the expected and documented behaviors. Ideally we'd fix that interface. But I don't know how to do that and this is better than nothing.

[KlausC]

As soon as the flisp part is fixed, the limits introduced in #53713 and here have to be removed.

[KlausC]

This PR is now finished and can be merged if no objections.