Ternary selection cleanup

Moved the ternary selection code to TParseContext
where checks were added to make sure arrays and
structs can't use it.

Change-Id: If3c007820870276cdf540005e095d89d54949bc4
Reviewed-on: https://swiftshader-review.googlesource.com/3669
Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>