This commit fixes saturated add/sub instructions in generic LLVM code
generation path.

Bug: b/115344057
Test: dEQP-GLES3.functional.texture.wrap.rgba8
Change-Id: Ie3e3b708565b3ad255804090e8a3ee5521f42982
Reviewed-on: https://swiftshader-review.googlesource.com/20928Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>
Tested-by: Chris Forbes <chrisforbes@google.com>