Fix min/max constant folding.

Change-Id: I9ac2051f5cc8703e804b86c0006bb1b9fe48f521
Reviewed-on: https://swiftshader-review.googlesource.com/5082
Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>