Accept null PhysReg arguments to checkRegMaskInterference.

Calling checkRegMaskInterference(VirtReg) checks if VirtReg crosses any
regmask operands, regardless of the registers they clobber.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@158563 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/CodeGen/LiveRegMatrix.cpp b/lib/CodeGen/LiveRegMatrix.cpp
index 61e1432..cdb1776 100644
--- a/lib/CodeGen/LiveRegMatrix.cpp
+++ b/lib/CodeGen/LiveRegMatrix.cpp
@@ -110,7 +110,7 @@
   // The BitVector is indexed by PhysReg, not register unit.
   // Regmask interference is more fine grained than regunits.
   // For example, a Win64 call can clobber %ymm8 yet preserve %xmm8.
-  return !RegMaskUsable.empty() && !RegMaskUsable.test(PhysReg);
+  return !RegMaskUsable.empty() && (!PhysReg || !RegMaskUsable.test(PhysReg));
 }
 
 bool LiveRegMatrix::checkRegUnitInterference(LiveInterval &VirtReg,