Don't use %g0 to materialize 0 directly.

The wired physreg doesn't work on tied operands like on MOVXCC.

Add a README note to fix this later.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@182225 91177308-0d34-0410-b5e6-96231b3b80d8