The PowerPC VRSAVE register has been somewhat of an odd beast since

the Altivec extensions were introduced.  Its use is optional, and
allows the compiler to communicate to the operating system which
vector registers should be saved and restored during a context switch.
In practice, this information is ignored by the various operating
systems using the SVR4 ABI; the kernel saves and restores the entire
register state.  Setting the VRSAVE register is no longer performed by
the AIX XL compilers, the IBM i compilers, or by GCC on Power Linux
systems.  It seems best to avoid this logic within LLVM as well.

This patch avoids generating code to update and restore VRSAVE for the
PowerPC SVR4 ABIs (32- and 64-bit).  The code remains in place for the
Darwin ABI.

llvm-svn: 165656