gallivm/ppc64le: adjust VSX code generation control.

gallivm/ppc64le: adjust VSX code generation control.

In lp_build_create_jit_compiler_for_module(), advance the minimum
version of LLVM for VSX code generation to 4.0; this is the minimum
revision at which several known VSX code generation bugs are fixed:

  https://llvm.org/bugs/show_bug.cgi?id=25503 (fixed in 3.8.1)
  https://llvm.org/bugs/show_bug.cgi?id=26775 (fixed in 3.8.1)
  https://llvm.org/bugs/show_bug.cgi?id=33531 (fixed in 4.0)

An llc performance bug introduced in LLVM 4.0,

  https://llvm.org/bugs/show_bug.cgi?id=34647

is still pending as of LLVM 5.0, but only has a pronounced effect on
one of the Piglit tests: ext_transform_feedback-max-varyings.

All changes tested via Piglit.

Cc: "17.2" <mesa-stable@lists.freedesktop.org>
Signed-off-by: Ben Crocker <bcrocker@redhat.com>
Acked-by: Roland Scheidegger <sroland@vmware.com>