i956: Add a function to load a 64-bit register from a buffer

i956: Add a function to load a 64-bit register from a buffer

Adds brw_load_register_mem64 which is similar to brw_load_register_mem
except that it queues two GEN7_MI_LOAD_REGISTER_MEM commands in order
to load both halves of a 64-bit register. The function is implemented
by splitting the 32-bit version into an internal helper function which
takes a size.

This will later be used to set the 64-bit predicate source registers.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>