Subzero: Use register availability during lowering to improve the code.

src/IceRegAlloc.cpp

Index: src/IceRegAlloc.cpp
diff --git a/src/IceRegAlloc.cpp b/src/IceRegAlloc.cpp
index dacbc009bda6846fed013fb345dd1d7621032918..aef3d4fecca9f8f7f607838c99a481a084e0c713 100644
--- a/src/IceRegAlloc.cpp
+++ b/src/IceRegAlloc.cpp
@@ -138,7 +138,7 @@ bool LinearScan::livenessValidateIntervals(
     const DefUseErrorList &DefsWithoutUses,
     const DefUseErrorList &UsesBeforeDefs,
     const CfgVector<InstNumberT> &LRBegin,
-    const CfgVector<InstNumberT> &LREnd) {
+    const CfgVector<InstNumberT> &LREnd) const {
   if (DefsWithoutUses.empty() && UsesBeforeDefs.empty())
     return true;