Add a -regalloc=default option that chooses a register allocator based on the -O
optimization level.

This only really affects llc for now because both the llvm-gcc and clang front
ends override the default register allocator. I intend to remove that code later.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@104904 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/CodeGen/LLVMTargetMachine.cpp b/lib/CodeGen/LLVMTargetMachine.cpp
index b584704..7f1a7c4 100644
--- a/lib/CodeGen/LLVMTargetMachine.cpp
+++ b/lib/CodeGen/LLVMTargetMachine.cpp
@@ -358,7 +358,7 @@
                    /* allowDoubleDefs= */ true);
 
   // Perform register allocation.
-  PM.add(createRegisterAllocator());
+  PM.add(createRegisterAllocator(OptLevel));
   printAndVerify(PM, "After Register Allocation");
 
   // Perform stack slot coloring and post-ra machine LICM.