tgsi: accept offsets for sample opcodes too in the text parser

tgsi: accept offsets for sample opcodes too in the text parser

sample opcodes are a little oddly represented in the opcode_info, since
they don't count as texture instructions - they don't have valid target
information, but they may have offsets (unlike "ordinary" texture
instructions, the texture token may be optional for them).
So just make sure with these opcodes the optional offsets are accepted.

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