Added support for the invariant keyword in the parser.
Also made sure all in/out types were handled properly
in OutputASM.cpp.

Change-Id: I40f0bc9caf5bccc691aa60dfaa90bed5d1d7d238
Reviewed-on: https://swiftshader-review.googlesource.com/3663Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>