Replace switch statements in instanced vertex shaders

Replaces switch statements with 3-way if’s. The switches as they were
seemed to be the cause of an Adreno compiler crash.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2136103011

Review-Url: https://codereview.chromium.org/2136103011
1 file changed