Minor unary math cleanup

The unary math related functions were already
added to TParseContext, but a few sections in
the parser hadn't been converted yet.

Change-Id: Idbc96e6e91acce28310f391a50d219e39dfc185b
Reviewed-on: https://swiftshader-review.googlesource.com/3668
Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>