legalizer/split.rs: simplify_branch_arguments: use SmallVec instead of Vec
legalizer/split.rs: simplify_branch_arguments: use SmallVec instead of Vec

This function is responsible for 8.5% of all heap allocation (calls) in CL.
This change avoids almost all of them by using a SmallVec::<[Value; 32]>
instead.  Dynamic instruction count falls by 0.25%.  The fixed size of 32 was
arrived at after profiling with fixed sizes of 1, 2, 4, 8, 16, 32, 64 and 128.
32 is as high as I can push it without the instruction count starting to creep
up again, and gets almost all the block-reduction win of 64 and 128.

stoklund/cretonne,stoklund/cretonne,stoklund/cretonne