gallivm: fix texture function name (key) when using txf/ld

gallivm: fix texture function name (key) when using txf/ld

When using the texel fetch functions rather than ordinary texturing,
the arguments are all int vecs instead of float vecs, not to mention
the actual function would look completely different. Hence this must
be included in the texture function name (which serves as the key)
otherwise things crash badly when a shader accesses the same texture
and sampler unit with both txf/ld and ordinary texturing instructions
with otherwise matching keys.