1. only do isSampler checking when update int uniforms
2. inline some simple methods of LinkUniform

This fix reduce CPU time spent on `isSampler` call from 70ms to 30ms in
T-Rex bench on specific platform.

Bug: angleproject:3743
Change-Id: I98ef3c892df27e08d54ed40946d924b5a50c796c
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1725114
Commit-Queue: Tobin Ehlis <tobine@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>