i965: Introduce the FIND_LIVE_CHANNEL pseudo-opcode.

i965: Introduce the FIND_LIVE_CHANNEL pseudo-opcode.

This instruction calculates the index of an arbitrary channel enabled
in the current execution mask.  It's expected to be used as input for
the BROADCAST opcode, but it's implemented as a separate instruction
rather than being baked into BROADCAST because FIND_LIVE_CHANNEL has
no dependencies so it can always be CSE'ed with other instances of the
same instruction within a basic block.

v2: Whitespace fixes.

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