Add a "nop filler" pass to SPU.

Filling no-ops is done just before emitting of assembly,
when the instruction stream is final. No-ops are inserted
to align the instructions so the dual-issue of the pipeline
is utilized. This speeds up generated code with a minimum of 
1% on a select set of algorithms.

This pass may be redundant if the instruction scheduler and 
all subsequent passes that modify the instruction stream 
(prolog+epilog inserter, register scavenger, are there others?)
are made aware of the instruction alignments.

llvm-svn: 123226