intel/compiler: fix 16-bit int brw_negate_immediate and brw_abs_immediate

intel/compiler: fix 16-bit int brw_negate_immediate and brw_abs_immediate

From Intel Skylake PRM, vol 07, "Immediate" section (page 768):

"For a word, unsigned word, or half-float immediate data,
software must replicate the same 16-bit immediate value to both
the lower word and the high word of the 32-bit immediate field
in a GEN instruction."

This fixes the int16/uint16 negate and abs immediates that weren't
taking into account the replication in lower and upper words.

v2: Integer cases are different to Float cases. (Jason Ekstrand)
    Included reference to PRM (Jose Maria Casanova)
v3: Make explicit uint32_t casting for left shift (Jason Ekstrand)
    Split half float implementation. (Jason Ekstrand)
    Fix brw_abs_immediate (Jose Maria Casanova)

Cc: "18.0 18.1" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>