intel/vec4: Fix lowering of multiplication by 16-bit constant

Existing code was ignoring whether the type of the immediate source
was signed or not.  If the source was signed, it would ignore small
negative values but it also would wrongly accept values between
INT16_MAX and UINT16_MAX, causing the atual value to later be
reinterpreted as a negative number (under 16-bits).

Fixes tests/shaders/glsl-mul-const.shader_test in Piglit for older
platforms that don't support MUL with 32x32 types and use vec4.

Cc: <mesa-stable@lists.freedesktop.org>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
(cherry picked from commit 766fdecc)