[llvm] [LLVM] Slay undead copysign code (PR #111269)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> I _believe_ the PowerPC target code has setOperationAction for ppc_f128 in a way that somehow still finds this codepath. 

It actually hits the default expand handling. ppc_fp128 surprisingly isn't a legal type:

```
Legalizing node: t20: ppcf128 = fcopysign t5, t4
Analyzing result type: ppcf128
Expand float result: t20: ppcf128 = fcopysign t5, t4
Creating new node: t23: i128 = bitcast t5
Creating new node: t24: i64 = extract_element t23, Constant:i64<1>
Creating new node: t25: i64 = extract_element t23, Constant:i64<0>
Creating new node: t26: f64 = bitcast t25
Creating new node: t27: f64 = bitcast t24

```

and it does result in the call:

```
	lfs 2, .LCPI0_0 at toc@l(3)
	bl copysignl
```

https://github.com/llvm/llvm-project/pull/111269