In previous logic, we didn't check the structure field member type. So
when passing the non-struct member of a structure to a function, it
would think that struct mapping was needed.
In this patch, we add more checking so that struct mapping only happens
when there are structure copy or passing a structure to a function.

BUG=angleproject:2967
Change-Id: Ic98e884c8f8540e180cdf40a0e036ffef18c1689
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1638227
Commit-Queue: Jiajia Qin <jiajia.qin@intel.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>