This merges two very similar pieces of code into one simpler function.
The function doesn't use any maps or indirection to build the merged
varyings list. It also fixes a potential bug with IO blocks and name
matching due to the code bifurcation.

Bug: angleproject:5496
Change-Id: Ibf54faeeb01d1940570b366ed153fff7c9135c52
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2606533
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>