Fix bit shift compilation

Bit shifts compilation was failing when the types were a mismatch,
but shifting a uint by an int (or vice versa) is allowed, so types
may not match, and it's fine, as long as both are integer types.

Change-Id: I86c52a572625b1d09803b0a8b887a63756544101
Reviewed-on: https://swiftshader-review.googlesource.com/3887
Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>