AArch64: fix bitcode upgrade of @llvm.neon.addp.

We were upgrading it to faddp, but a version taking two type parameters instead
of one. This then got upgraded a second time to the version with just one
parameter, but occasionally (for reasons I don't understand) this unusual
two-stage process corrupted a use-list, leading to a crash when the two faddp
declarations didn't match.