gallivm: Fix saturated signed psub/padd intrinsics on llvm 8

LLVM 8 did remove both the signed and unsigned sse2/avx intrinsics in
the end, and provide arch-independent llvm intrinsics instead.
Fixes a crash when using snorm framebuffers (tested with piglit
arb_color_buffer_float-render GL_RGBA8_SNORM -auto).

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
CC: <mesa-stable@lists.freedesktop.org>
(cherry picked from commit 045f05a2)
Conflicts resolved by Dylan Baker