gallivm/tgsi: fix src modifier fetching with non-float types.

gallivm/tgsi: fix src modifier fetching with non-float types.

Need to take the type into account. Also, if we want to allow
mov's with modifiers we need to pick a type (assume float).

v2: don't allow all modifiers on all type, in particular don't allow
absolute on non-float types and don't allow negate on unsigned.
Also treat UADD as signed (despite the name) since it is used
for handling both signed and unsigned integer arguments and otherwise
modifiers don't work.
Also add tgsi docs clarifying this.

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>