[llvm] [AArch64][GISel] Assign registers into FPR if they feed into FP instructions indirectly via PHI (PR #94618)

[Thorsten Schütt via llvm-commits]

tschuett wrote:

```
enum class RegisterBank { GPR, FPR, Unknown};

RegisterBank isUsedBy(const MachineInstr& MI);

```
Hoist the if and phi and select and more checking into a generic function that might be useful in many places.

Which RegisterBank uses this phi? Which RegisterBank uses this select? Which RegisterBank uses this G_LOAD?

https://github.com/llvm/llvm-project/pull/94618