[llvm] [GlobalISel][ARM] Legalization of G_CONSTANT using constant pool (PR #98308)

[Thorsten Schütt via llvm-commits]

tschuett wrote:

https://github.com/llvm/llvm-project/pull/97513 showed that the standard `isLegalOrBeforeLegalizer` does not work with `customFor`.

Arm can select s32 and p0. It is up to the selector find out how.

`legalFor` is preferably over `customFor`.

We signed a declaration of independence. :-)

https://github.com/llvm/llvm-project/pull/98308