After removal of Darwin as a PowerPC subtarget, the VRSAVE save/restore/spill/update code is dead, so remove it while keeping support for vrsave and related instruction aliases for inline asm. I've pre-commited tests to document the existing vrsave handling in relation to @llvm.eh.unwind.init and inline asm usage to show this patch doesn't change the exiting behavior.
Previous patch was not really NFC, since we were inserting VRSAVE pseudos on AIX, then emiting a fatal error in frame lowering. This was because we missed changing the condition gating the vrsave insertion to include the AIX abi.
I've landed a patch which shows the previous error related to VRSAVE, and the new error I've added in LowerReturn to replace it. The new behavior is more lenient in that we can use vector instructions inside a functions body, and will now only error if we try to pass or return vector types.