Only enable 64-bit bswap DAG combines for PPC64

Compiling in 32-bit mode on a P7 would assert after 64-bit DAG combines were
added for bswap with load/store. This is because these combines are really only
valid in 64-bit mode, regardless of the CPU (and this was not being checked).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@178286 91177308-0d34-0410-b5e6-96231b3b80d8