For live ranges with many use positions, such as those encountered in
some unity asm.js code, this change significantly reduces compile time
(e.g. benchmarks/Compile/slow_nbody1.js: from ~6s to 2s). The
improvement is solely due to regressions (fixed by this CL) due to
splintering.

This CL does not fully address compile time problems for large
functions in Turbofan, but constitutes a step in the right direction.

Review URL: https://codereview.chromium.org/1386253004

Cr-Commit-Position: refs/heads/master@{#31220}