The assert() can be safely removed. This was tested by doing the
following on my Linux machine.

  1) remove the assert() in third_party/swiftshader/third_party/subzero/src/IceTargetLoweringX86BaseImpl.h:6491
  2) change the 11 into 1 in src/Reactor/ReactorUnitTests.cpp:492 (to ensure full coverage of the Blend operation)
  3) cd build && cmake .. -DCMAKE_BUILD_TYPE=Debug -DREACTOR_BACKEND=Subzero && make -j32 && ./ReactorUnitTests

Without step 1), ReactorUnitTests crasehs with an assertion in debug
builds only.

Bug: None
Change-Id: I3ab3600eee0bb7a9c9a1bfc3fe091e24c4dfc366
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/39053
Kokoro-Presubmit: David Turner <digit@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>