i965/vec4: fill src_reg type using the constructor type parameter

i965/vec4: fill src_reg type using the constructor type parameter

The src_reg constructor that received the glsl_type was using it
only to build the swizzle, but not to fill this->type as dst_reg
is doing.

This caused some type mismatch between movs and alu operations
on the NIR path, so copy propagation optimization was not applied
to remove unneeded movs if negate modifier was involved. This was
first detected on minus (negate+add) operations.

Shader DB results (taking into account only vec4):

total instructions in shared programs: 20019 -> 19934 (-0.42%)
instructions in affected programs:     2918 -> 2833 (-2.91%)
helped:                                79
HURT:                                  0
GAINED:                                0
LOST:                                  0

Reviewed-by: Matt Turner <mattst88@gmail.com>