CodeGen: Tune the SmallVector size in LiveRange

The vast majority of LiveRanges (ie, 4/5) have exactly 1 segment and 1
value number, and a good chunk of the rest have 2 of each, so
allocating space for 4 is wasteful. This is especially noticeable when
dealing with a very large number of vregs, and I have an internal case
where dropping this to 2 shaves over 5% off of peak memory when
compiling a particularly large function.

llvm-svn: 262681