Fix several compilation warnings.

Change-Id: I6525eab19ebe5a90e8f0fbfbef3d3e2ba2170da9
Reviewed-on: https://swiftshader-review.googlesource.com/2944
Reviewed-by: Greg Hartman <ghartman@google.com>
Tested-by: Greg Hartman <ghartman@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>