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

[Thorsten Schütt via llvm-commits]

tschuett wrote:

Neutral version:
This PR lowers to constant pool loads if:
```
if (ConstantMaterializationCost(ImmVal, ST) > 2 &&
        !ST->genExecuteOnly())
```
using the standard legalizer tools.

https://github.com/llvm/llvm-project/pull/96225 does the same manually in the selector pass.

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