nir: coverity sign-extension fix

nir: coverity sign-extension fix

Not 100% sure, but I think being an unsigned literal will help:

CID 1358505 (#1 of 1): Unintended sign extension
(SIGN_EXTENSION)sign_extension: Suspicious implicit sign extension:
load1->def.num_components with type unsigned char (8 bits, unsigned) is
promoted in load1->def.num_components * (load1->def.bit_size / 8) to
type int (32 bits, signed), then sign-extended to type unsigned long (64
bits, unsigned). If load1->def.num_components * (load1->def.bit_size /
8) is greater than 0x7FFFFFFF, the upper bits of the result will all be
1.

Signed-off-by: Rob Clark <robclark@freedesktop.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>