tgsi: fix regression in indexed const lookups

tgsi: fix regression in indexed const lookups

This function was calling get_input_base() and get_output_base() to
get the names of a couple of register to use as temps.  Those
functions no longer return registers, so adjust it to get the
registers elsewhere.

This change doesn't address the issue that it's a fairly poor way to
grab a register name by calling a function with an apparently
unrelated meaning.