intel/compiler: set correct precision fields for 3-source float instructions

intel/compiler: set correct precision fields for 3-source float instructions

Source0 and Destination extract the floating-point precision automatically
from the SrcType and DstType instruction fields respectively when they are
set to types :F or :HF. For Source1 and Source2 operands, we use the new
1-bit fields Src1Type and Src2Type, where 0 means normal precision and 1
means half-precision. Since we always use the type of the destination for
all operands when we emit 3-source instructions, we only need set Src1Type
and Src2Type to 1 when we are emitting a half-precision instruction.

v2:
 - Set the bit separately for each source based on its type so we can
   do mixed floating-point mode in the future (Topi).

v3:
 - Use regular citation style for the comment referencing the PRM (Matt).
 - Decided not to add asserts in the emission code to check that only
   mixed HF/F types are used since such checks would break negative tests
   for brw_eu_validate.c (Matt)

Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Matt Turner <mattst88@gmail.com>