[llvm] [SDAG] Add missing ppc_fp128 ExpandFloatRes for sincos[pi] (PR #128514)

[Benjamin Maxwell via llvm-commits]

================
@@ -49,3 +49,49 @@ define { ppc_fp128, ppc_fp128 } @test_sincospi_ppcf128(ppc_fp128 %a) {
   %result = call { ppc_fp128, ppc_fp128 } @llvm.sincospi.ppcf128(ppc_fp128 %a)
   ret { ppc_fp128, ppc_fp128 } %result
 }
+
+; FIXME: Recognise this as a tail call and omit the stack frame:
----------------
MacDue wrote:

Right, but returning a struct would never result in a tail call for the standard expansion. 
This IR does result in something that codegen _could_ reconguise as a tail call (and there's logic in SDAG that could do this), but right now it fails to do so. Look at the current codegen:

```
; CHECK-LABEL: test_sincos_ppcf128_tail_call:
; CHECK:       # %bb.0:
<frame setup>
; CHECK-NEXT:    mflr r0
; CHECK-NEXT:    stdu r1, -32(r1)
; CHECK-NEXT:    std r0, 48(r1)
; CHECK-NEXT:    .cfi_def_cfa_offset 32
; CHECK-NEXT:    .cfi_offset lr, 16

<call to sincos>
; CHECK-NEXT:    bl sincosl

<frame destruction> 
; CHECK-NEXT:    nop
; CHECK-NEXT:    addi r1, r1, 32
; CHECK-NEXT:    ld r0, 16(r1)
; CHECK-NEXT:    mtlr r0
; CHECK-NEXT:    blr
```

This could simply be a jump to `sincosl`. 

https://github.com/llvm/llvm-project/pull/128514