[PowerPC] Do not use vectors to codegen bswap with Altivec turned off

We have efficient codegen on P9 for lowering bswap that involves moving
the value into a vector reg and moving it back. However, the check under
which we custom lowered it did not adequately reflect the actual requirements.
It required only that the subtarget be an implementation of ISA 3.0 since all
compliant implementations have to provide the vector instructions.
However, the kernel builds have a valid use case for -mno-altivec -mcpu=pwr9
(i.e. don't emit vector code, don't have to save vector regs for context
switch). So we should require the correct features for this lowering.
Fixes https://bugs.llvm.org/show_bug.cgi?id=39334

llvm-svn: 347376