[PowerPC] Don't combine to bswap store on 1-byte truncating store

Turns out that there was no check for a store that truncates down
to a single byte when combining a (store (bswap...)) into a byte-swapping
store. This patch just adds that check.

Fixes https://bugs.llvm.org/show_bug.cgi?id=39478.

llvm-svn: 347288