gallivm: fix cube map array (and cube map shadow with bias) handling

gallivm: fix cube map array (and cube map shadow with bias) handling

In particular need to handle TEX2/TXB2/TXL2 opcodes.
cube map shadow with bias already used TXB2 which didn't work before
at all, despite that there's by default no piglit change (but using
no_quad_lod and no_rho_opt indeed passes some more tex-miplevel-selection
tests).
The actual sampling code still won't handle cube map arrays.

Reviewed-by: Brian Paul <brianp@vmware.com>