Allow remat of LI on PPC.

Allow load-immediates to be rematerialised in the register coalescer for
PPC. This makes test/CodeGen/PowerPC/big-endian-formal-args.ll fail,
because it relies on a register move getting emitted. The immediate load is
equivalent, so change this test case.

Patch by Tobias von Koch.

llvm-svn: 162727