remove unreachable samples for non-N32 imageshaders

CQ_INCLUDE_TRYBOTS=skia.primary:Test-Debian9-GCC-GCE-CPU-AVX2-x86_64-Release-SKNX_NO_SIMD
Bug: skia:
Change-Id: I2d9a22b22d72c81a742b8fd497797bff8174915b
Reviewed-on: https://skia-review.googlesource.com/21264
Commit-Queue: Eric Boren <borenet@google.com>
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Florin Malita <fmalita@chromium.org>
4 files changed