rustc_apfloat: fix typo in is_denormal

[klensy]

Sources:
https://github.com/llvm-mirror/llvm/blob/23efab2bbd424ed13495a420ad8641cb2c6c28f9/lib/Support/APFloat.cpp#L4341..L4346

[rust-highfive]

Changes rustc_apfloat. rustc_apfloat is currently in limbo and you almost certainly don't want to change it (see #55993).

cc @eddyb

[rust-highfive]

r? @estebank

(rust-highfive has picked a reviewer for you, use r? to override)

[eddyb]

Looks like this was a porting mistake of mine. But also, we don't even use the PPC code.
I'm torn. A bit weird that tests didn't catch this.

I'll go poke the people dealing with the licensing limbo just to make sure approving this tiny PR won't make anything slightly worse.

[bstrie]

@eddyb Per the file that @klensy linked, they derived this change from a file that is licensed under The University of Illinois Open Source License, which means that we should assume this change must be licensed as the same. Since you originally ported rustc-apfloat from a codebase under the same license, this doesn't change anything about the licensing of rustc-apfloat.

[Dylan-DPC-zz]

closing this as there's no much interest in this atm, we can revisit it some other time. Thanks