[PowerPC] Fix strict load-conversion recognition

Direct-move instructions are usually more efficient than load then store
for conversion. But direct moves are not needed when the source register
was just loaded from some address.

The pattern has already been recognized, but the source value of strict
nodes are not the first (that's the chain), but the second.

Reviewed By: shchenz

Differential Revision: https://reviews.llvm.org/D138011