i965/fs: Fix stride for immediate registers.

i965/fs: Fix stride for immediate registers.

When the width field was removed from fs_reg the BROADCAST handling
code in opt_algebraic() started to miss a number of trivial
optimization cases resulting in the ugly indirect-addressing sequence
to be emitted unnecessarily for some variable-indexed texturing and
UBO loads regardless of one of the sources of BROADCAST being
immediate.  Apparently the reason was that we were setting the stride
field to one for immediates even though they are typically uniform.
Width used to be set to one too which is why this optimization used to
work previously until the "reg.width == 1" check was removed.

The stride field of vector immediates is intentionally left equal to
one, because they are strictly speaking not uniform.  The assertion in
fs_generator makes sure that immediates have the expected stride as
consistency check.

Reviewed-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>