i965/fs: Fix LOAD_PAYLOAD handling in register coalesce is_nop_mov().

i965/fs: Fix LOAD_PAYLOAD handling in register coalesce is_nop_mov().

is_nop_mov() was broken for LOAD_PAYLOAD instructions in two ways: On
the one hand the original destination register offset wasn't being
taken into account which would give incorrect results if it was
already non-zero, and on the other hand all source registers were
being treated as if they had a size of 32B, which is almost never the
case in SIMD16 programs for non-header sources.

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>