i965/fs: Detect multi-register MOVs correctly in register_coalesce.

i965/fs: Detect multi-register MOVs correctly in register_coalesce.

register_coalesce() was considering the exec_size of the MOV
instruction alone to decide whether the register at offset+1 of the
source VGRF was being copied to inst->dst.reg_offset+1 of the
destination VGRF, which is only a valid assumption if the move has a
32-bit execution type.  Use regs_read() instead to find out the number
of registers copied by the instruction.

Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>