i965: Define virtual instruction to calculate the high 32 bits of a multiply.

i965: Define virtual instruction to calculate the high 32 bits of a multiply.

This instruction will translate to the MUL/MACH sequence that computes
the high 32-bits of the result of a 64-bit multiply.  Before Gen8
integer operations that used the accumulator were limited to 8-wide,
but the SIMD lowering pass can easily be hooked up to sidestep this
limitation, we just need a virtual opcode to represent the MUL/MACH
sequence in the IR.

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