Subzero: Improve regalloc performance by optimizing UnhandledPrecolored.

A lot of time was being spent in the two loops that check precolored ranges in the Unhandled set, specifically in the endsBefore() check.

Solve this by keeping a shadow copy of Unhandled, restricted to the ranges that are precolored.

BUG= none
R=jvoung@chromium.org

Review URL: https://codereview.chromium.org/622553003

src/IceRegAlloc.h

diff --git a/src/IceRegAlloc.h b/src/IceRegAlloc.h
index 791b349..99ed908 100644
--- a/src/IceRegAlloc.h
+++ b/src/IceRegAlloc.h
@@ -71,6 +71,9 @@
   typedef std::set<LiveRangeWrapper, RangeCompare> OrderedRanges;
   typedef std::list<LiveRangeWrapper> UnorderedRanges;
   OrderedRanges Unhandled;
+  // UnhandledPrecolored is a subset of Unhandled, specially collected
+  // for faster processing.
+  OrderedRanges UnhandledPrecolored;
   UnorderedRanges Active, Inactive, Handled;
   LinearScan(const LinearScan &) = delete;
   LinearScan &operator=(const LinearScan &) = delete;