Struct mapping consumes lots of time when the array lengh is very large.
And it's done unconditionally. This CL does struct mapping only if it's
a struct assignment or passing the struct to a function.

In the next step, we can use a function to do struct mapping instead of
struct array initialization.

Bug: angleproject:2967

Change-Id: Ie5a74fa05dbc0255ad664d6265dbb880a624f01e
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1612978Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jiajia Qin <jiajia.qin@intel.com>