[x86] make peekThroughBitcasts() a helper function

This should be hoisted further up so it can be used in DAGCombiner and other backends,
but I'm limiting the scope in the interest of patch minimalism.

It's not quite NFC because some of the replaced code was using an 'if' check rather
than a 'while' loop, so those cases would only look through a single bitcast.

llvm-svn: 264186