i965: Fix undefined df bits in brw_reg comparisons.

i965: Fix undefined df bits in brw_reg comparisons.

Commit 5310bca024f77da40ea6f4c275455f9cb0528f9e added a new "double df"
field to the brw_reg struct, adding an extra 4 bytes of data that isn't
usually initialized (or may contain irrelevant garbage if the struct is
mutated).  This means that it's no longer safe to memcmp().

Instead, add a brw_regs_equal() function which ignores the extra df bits
unless they matter.  To keep the implementation cheap, we wrap the first
set of fields in a union/struct so that we can use a single DWord
comparison.

v2: Drop unnecessary casts (caught by Francisco Jerez).

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>
Reviewed-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>