ConvertVertex's UintToUint and HalfFloatToHalfFloat were generating
identical shaders.

Bug: angleproject:3802
Change-Id: I8eb2d55debbc5501f191830a2723b02d0d1f0827
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1925248Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Hyunchang Kim <hckim.kim@samsung.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>