This makes the check a bit faster, by inlining the prefix check.
Also some cleanups to ValidateVertexShaderAttributeTypeMatch.

BUG=angleproject:2202

Change-Id: Ifeab4cd85a91a1639a461f44776a68ac98c5bd79
Reviewed-on: https://chromium-review.googlesource.com/761240Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>