Comment # 2 on bug 92709 from Matt Arsenault

(In reply to Michel Dänzer from comment #1)
> This happens since Mesa commit d72a26ec ("radeonsi: don't emit AMDGPU
> intrinsics for EX2, ROUND, TRUNC") because the LLVM function
> LibCallSimplifier::optimizeExp2() replaces the llvm.exp2.f32 call (which
> could be handled by the AMDGPU backend AFAICT) with an ldexpf one.

This is one of those annoying cases where ldexp doesn't have an equivalent
generic ISD node. We should probably add one and use it for the libcall and
handle it like most of the other math LibFuncs.