[PowerPC][Altivec] Fix constant argument for vec_dss

Summary:
This is similar to vec_ct* in https://reviews.llvm.org/rL304205.

The argument must be a constant, otherwise instruction selection
will fail. always_inline is not enough for isel to always fold
everything away at -O0.

The fix is to turn the function into macros in altivec.h.

Fixes https://bugs.llvm.org/show_bug.cgi?id=43072

Reviewers: nemanjai, hfinkel, #powerpc, wuzish

Reviewed By: #powerpc, wuzish

Subscribers: wuzish, kbarton, MaskRay, shchenz, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D66699

llvm-svn: 370902