st/glsl_to_tgsi: make samplers_used an uint32_t (v2)

st/glsl_to_tgsi: make samplers_used an uint32_t (v2)

It is used as a bitfield, so it seems cleaner to keep it unsigned.

The literal 1 is a (signed) int, and shifting into the sign bit is undefined
in C, so change occurences of 1 to 1u.

v2: add an assert for bitfield size and use 1u << idx

Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com> (v1)
Reviewed-by: Marek Olšák <marek.olsak@amd.com> (v1)