glsl: Add a lowering pass to turn complicated UBO references to vector loads.

glsl: Add a lowering pass to turn complicated UBO references to vector loads.

v2: Reduce the impenetrable code in emit_ubo_loads() by 23 lines by keeping
    the ir_variable as the variable part of the offset from handle_rvalue(),
    and track the constant offsets from that with a plain old integer value,
    avoiding a bunch of temporary variables in the array and struct handling.
    Also, fix file description doxygen.
v3: Fix a row vs col typo, and fix spelling in a comment.

Reviewed-by: Eric Anholt <eric@anholt.net>