glsl/types: Make is_integer() make more sense

Merged Faith Ekstrand requested to merge gfxstrand/mesa:review/glsl-type-is-integer into master

When you read type->is_integer() in code, you expect that it's checking whether or not the type is an integer type. In particular, you would expect it to return true for uint64_t which it doesn't. This little MR attempts to fix the confusion.

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