Don't needlessly traverse the left hand side of assignments.

Don't needlessly traverse the left hand side of assignments.

Previously we processed assignments in PostVisit, i.e. after both the
left and right side had been traversed. This produces temporaries for
the left hand side, which we don't use since we want to assign to the
lvalue. So instead we can explicitly traverse the right hand side, and
for the left hand side only traverse indirect indexing expressions.

Change-Id: I9ec0596a9c256921b65a9f70428d950959f66aa0
Reviewed-on: https://swiftshader-review.googlesource.com/13630
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>