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

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