simplify neon shifts

These still generate vshr/vshl with immediates with both GCC and Clang.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2194953002
CQ_INCLUDE_TRYBOTS=master.client.skia:Test-Ubuntu-GCC-GCE-CPU-AVX2-x86_64-Release-SKNX_NO_SIMD-Trybot

Based on https://codereview.chromium.org/2196773002

Review-Url: https://codereview.chromium.org/2194953002
1 file changed