i965/fs: Disable CSE on instructions writing to HW_REG.

i965/fs: Disable CSE on instructions writing to HW_REG.

CSE would otherwise combine the two mul(8) emitted by [iu]mulExtended:

	mul(8)  acc0 x y
	mach(8) null x y
	mov(8)  lsb  acc0
	...
	mul(8)  acc0 x y
	mach(8) msb  x y
Into:
	mul(8)  temp x y
	mov(8)  acc0 temp
	mach(8) null x y
	mov(8)  lsb  acc0
	...
	mov(8)  acc0 temp
	mach(8) msb  x y

But mul(8) into the accumulator produces more than 32-bits of precision,
which is required and lost if multiplying into a general register and
moving to the accumulator.

Reviewed-by: Eric Anholt <eric@anholt.net>