LLVM 8 removes several x86 SSE intrinsics for saturated arithmetic, but
provides target-independent replacements. Note there are now three code
paths for these vector operations:
- With LLVM 8+, use the new target-independent intrinsics.
- With LLVM 7 on x86, use the SSE intrinsics.
- On other architectures, lower to a generic sequence of vector
  operations (lowerPSAT).

Bug: b/139412871
Change-Id: I849f8b5c004849c628507d6377779df170a11d93
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/36788
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>