[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:

```
// Check if that load feeds fp instructions.
    // In that case, we want the default mapping to be on FPR
    // instead of blind map every scalar to GPR.
    if (any_of(MRI.use_nodbg_instructions(MI.getOperand(0).getReg()),
               [&](const MachineInstr &UseMI) {
                 // If we have at least one direct use in a FP instruction,
                 // assume this was a floating point load in the IR. If it was
                 // not, we would have had a bitcast before reaching that
                 // instruction.
                 if (isa<GPHi>(&UseMI)
                   if (isPHIWithFPContraints(UseMI))
                     return true;
                 //
                 // Int->FP conversion operations are also captured in
                 // onlyDefinesFP().
                 return onlyUsesFP(UseMI, MRI, TRI) ||
                        onlyDefinesFP(UseMI, MRI, TRI);
               }))
      OpRegBankIdx[0] = PMI_FirstFPR;
    break;
```

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